Copy of word_list_common (version 1)csv v1.7 Flashcards
avoid
stay clear from; keep away from; keep out of the way of someone or something; “Her former friends now avoid her”prevent the occurrence of; prevent from happening; “Let’s avoid a confrontation”; “head off a confrontation”; “avert a strike”refrain from doing something; “She refrains from calling her therapist too often”; “He should avoid publishing his wife’s memories”refrain from certain foods or beverages; “I keep off drugs”; “During Ramadan
avoidance
deliberately avoiding; keeping away from or preventing from happening
avoirdupois
a system of weights based on the 16-ounce pound (or 7
avouch
admit openly and bluntly; make no bones about
avow
to declare or affirm solemnly and formally as true; “Before God I swear I am innocent”admit openly and bluntly; make no bones about
avowal
a statement asserting the existence or the truth of something
avowed
openly declared as such; “an avowed enemy”; “her professed love of everything about that country”; “McKinley was assassinated by a professed anarchist”
avulsion
an abrupt change in the course of a stream that forms the boundary between two parcels of land resulting in the loss of part of the land of one landowner and a consequent increase in the land of anothera forcible tearing or surgical separation of one body part from another
avuncular
resembling a uncle in kindness or indulgencebeing or relating to an uncle
await
look forward to the probable occurrence of; “We were expecting a visit from our relatives”; “She is looking to a promotion”; “he is waiting to be drafted”
awake
not in a state of sleep; completely conscious; “lay awake thinking about his new job”; “still not fully awake”mentally perceptive and responsive;”an alert mind”; “alert to the problems”; “alive to what is going on”; “awake to the dangers of her situation”; “was now awake to the reality of his predicament”
awaken
cause to become awake or conscious; “He was roused by the drunken men in the street”; “Please wake me at 6 AM.”stop sleeping; “She woke up to the sound of the alarm clock”make aware; “They were awakened to the sad facts”
awakening
the act of waking; “it was an early awakening”; “it was the waking up he hated most”
award
give
aware
(sometimes followed by `of’) having or showing knowledge or understanding or realization or perception; “was aware of his opponent’s hostility”; “became aware of her surroundings”; “aware that he had exceeded the speed limit”bearing in mind; attentive to; “ever mindful of her health”; “mindful of his responsibilities”; “mindful of these criticisms
awash
covered with water; “the main deck was afloat (or awash)”; “the monsoon left the whole place awash”; “a flooded bathroom”; “inundated farmlands”; “an overflowing tub”
away
not present; having left; “he’s away right now”; “you must not allow a stranger into the house when your mother is away”used of an opponent’s ground; “an away game”(of a baseball pitch) on the far side of home plate from the batter; “the pitch was away (or wide)”; “an outside pitch”
awe
inspire awe in; “The famous professor awed the undergraduates”
awe-inspiring
inspiring awe or admiration or wonder; “New York is an amazing city”; “the Grand Canyon is an awe-inspiring sight”; “the awesome complexity of the universe”; “this sea
awed
inspired by a feeling of fearful wonderment or reverence; “awed by the silence”; “awful worshippers with bowed heads”having or showing a feeling of mixed reverence and respect and wonder and dread; “stood in awed silence before the shrine”; “in grim despair and awestruck wonder”
aweigh
(used of an anchor) hanging clear of the bottom; “anchors aweigh”(of an anchor) just clear of the bottom
aweless
devoid of any feeling of awe or reverenceneither feeling nor showing respect
awesome
inspiring awe or admiration or wonder; “New York is an amazing city”; “the Grand Canyon is an awe-inspiring sight”; “the awesome complexity of the universe”; “this sea
awestricken
having or showing a feeling of mixed reverence and respect and wonder and dread; “stood in awed silence before the shrine”; “in grim despair and awestruck wonder”
awful
exceptionally bad or displeasing; “atrocious taste”; “abominable workmanship”; “an awful voice”; “dreadful manners”; “a painful performance”; “terrible handwriting”; “an unspeakable odor came sweeping into the room”causing fear or dread or terror; “the awful war”; “an awful risk”; “dire news”; “a career or vengeance so direful that London was shocked”; “the dread presence of the headmaster”; “polio is no longer the dreaded disease it once was”; “a dreadful storm”; “a fearful howling”; “horrendous explosions shook the city”; “a terrible curse”offensive or even (of persons) malicious; “in a nasty mood”; “a nasty accident”; “a nasty shock”; “a nasty smell”; “a nasty trick to pull”; “Will he say nasty things at my funeral?”- Ezra Poundinspired by a feeling of fearful wonderment or reverence; “awed by the silence”; “awful worshippers with bowed heads”extreme in degree or extent or amount or impact; “in a frightful hurry”; “spent a frightful amount of money”inspiring awe or admiration or wonder; “New York is an amazing city”; “the Grand Canyon is an awe-inspiring sight”; “the awesome complexity of the universe”; “this sea
awfully
used as intensifiers; “terribly interesting”; “I’m awful sorry”of a dreadful kind; “there was a dreadfully bloody accident on the road this morning”in a terrible manner; “she sings terribly”
awhile
for a short time; “sit down and stay awhile”; “they settled awhile in Virginia before moving West”; “the baby was quiet for a while”
awkward
causing inconvenience; “they arrived at an awkward time”lacking grace or skill in manner or movement or performance; “an awkward dancer”; “an awkward gesture”; “too awkward with a needle to make her own clothes”; “his clumsy fingers produced an awkward knot”difficult to handle or manage especially because of shape; “an awkward bundle to carry”; “a load of bunglesome paraphernalia”; “clumsy wooden shoes”; “the cello
awl
a pointed tool for marking surfaces or for punching small holes
awlwort
small aquatic plant having tufted awl-shaped leaves in a basal rosette and minute white flowers; circumboreal
awn
slender bristlelike appendage found on the bracts of grasses
awning
a canopy made of canvas to shelter people or things from rain or sun
awry
turned or twisted toward one side; “a…youth with a gorgeous red necktie all awry”- G.K.Chesterton; “his wig was
ax
chop or split with an ax; “axe wood”terminate; “The NSF axed the research program and stopped funding it”
axe
chop or split with an ax; “axe wood”terminate; “The NSF axed the research program and stopped funding it”
axenic
(of experimental animals) raised under sterile conditions; “axenic conditions”; “germfree animals”(used of cultures of microorganisms) completely free from other organisms; “an axenic culture”
axial
of or relating to or resembling an axis of rotationrelating to or attached to the axis; “axial angle”situated on or along or in the direction of an axis
axial skeleton
the part of the skeleton that includes the skull and spinal column and sternum and ribs
axil
the upper angle between an axis and an offshoot such as a branch or leafstalk
axilla
the hollow under the arm where it is joined to the shoulder; “they were up to their armpits in water”
axillary
of or relating to the axilof or relating to the armpit; “axillary gland”
axiology
the study of values and value judgments
axiom
a saying that is widely accepted on its own merits(logic) a proposition that is not susceptible of proof or disproof; its truth is assumed to be self-evident
axiomatic
evident without proof or argument; “an axiomatic truth”; “we hold these truths to be self-evident”containing aphorisms or maxims; “axiomatic wisdom”of or relating to or derived from axioms; “axiomatic physics”; “the postulational method was applied to geometry”- S.S.Stevens
axis
a straight line through a body or figure that satisfies certain conditionsthe main stem or central part about which plant organs or plant parts such as branches are arrangedin World War II the alliance of Germany and Italy in 1936 which later included Japan and other nations; “the Axis opposed the Allies in World War II”a group of countries in special alliancethe 2nd cervical vertebra; serves as a pivot for turning the headthe center around which something rotates
axle
a shaft on which a wheel rotates
axletree
a dead axle on a carriage or wagon that has terminal spindles on which the wheels revolve
axolotl
larval salamander of mountain lakes of Mexico that usually lives without metamorphosing
axon
long nerve fiber that conducts away from the cell body of the neuron
axseed
European herb resembling vetch; naturalized in the eastern United States; having umbels of pink-and-white flowers and sharp-angled pods
ayah
(in India) a native nursemaid who looks after children
aye-aye
nocturnal lemur with long bony fingers and rodent-like incisor teeth closely related to the lemurs
ayin
the 16th letter of the Hebrew alphabet
azalea
any of numerous ornamental shrubs grown for their showy flowers of various colors
azedarach
tree of northern India and China having purple blossoms and small inedible yellow fruits; naturalized in the southern United States as a shade tree
azide
a chemical compound containing the azido group combined with an element or radical
azimuth
the azimuth of a celestial body is the angle between the vertical plane containing it and the plane of the meridian
azo
relating to or containing the azo radical
azo dye
any dye containing one or more azo groups
azoic
before the appearance of life; “azoic rocks contain not organic remains”
azote
an obsolete name for nitrogen
azotemia
accumulation in the blood of nitrogenous waste products (urea) that are usually excreted in the urine
azotic
of or containing nitrogen; “nitric acid”
azure
of a deep somewhat purplish blue color similar to that of a clear October sky; “October’s bright blue weather”
azurite
blue carbonate of copper; blue malachite
azygous
occurring singly; not one of a pair; “the azygous muscle of the uvula”
b
aerobic rod-shaped spore-producing bacterium; often occurring in chainlike formations; found primarily in soiloriginally thought to be a single vitamin but now separated into several B vitaminsa trivalent metalloid element; occurs both in a hard black crystal and in the form of a yellow or brown powdera logarithmic unit of sound intensity equal to 10 decibels(physics) a unit of nuclear cross section; the effective circular area that one particle presents to another as a target for an encounterthe 2nd letter of the Roman alphabetthe blood group whose red cells carry the B antigen
b.o.
malodorousness resulting from a failure to bathe
baa
cry plaintively; “The lambs were bleating”
baba
a small cake leavened with yeast
baba au rhum
a baba soaked in rum
babassu
tall feather palm of northern Brazil with hard-shelled nuts yielding valuable oil and a kind of vegetable ivory
babbitt
line with a Babbitt metal
babble
utter meaningless sounds
babbler
an obnoxious and foolish and loquacious talkerany of various insectivorous Old World birds with a loud incessant song; in some classifications considered members of the family Muscicapidae
babbling
gibberish resembling the sounds of a baby
babe
a very young child (birth to 1 year) who has not yet begun to walk or talk; “the baby began to cry again”; “she held the baby in her arms”; “it sounds simple
babies’-breath
tall plant with small lance-shaped leaves and numerous tiny white or pink flowers
babirusa
Indonesian wild pig with enormous curved canine teeth
baboon
large terrestrial monkeys having doglike muzzles
babu
used as a Hindi courtesy title; equivalent to English `Mr’
babushka
a woman’s headscarf folded into a triangle and tied under the chin; worn by Russian peasant women
baby
treat with excessive indulgence; “grandparents often pamper the children”; “Let’s not mollycoddle our students!”
baby carriage
a small vehicle with four wheels in which a baby or child is pushed around
baby grand
a small grand piano
baby talk
an adult’s imitation of the speech of a young childthe developing speech of a young child
baby tooth
one of the first temporary teeth of a young mammal (one of 20 in children)
baby-sit
take watchful responsibility for; “I baby-sit the neighbor’s plants when she is out of town”work or act as a baby-sitter; “I cannot baby-sit tonight; I have too much homework to do”
baby-sitter
a person engaged to care for children when the parents are not home
baccalaureate
a farewell sermon to a graduating class at their commencement ceremoniesan academic degree conferred on someone who has successfully completed undergraduate studies
baccarat
a card game played in casinos in which two or more punters gamble against the banker; the player wins who holds 2 or 3 cards that total closest to nine
baccate
resembling a berryproducing or bearing berries
bacchanal
used of riotously drunken merrymaking; “a night of bacchanalian revelry”; “carousing bands of drunken soldiers”; “orgiastic festivity”
bacchanalia
an orgiastic festival in ancient Greece in honor of Dionysus (= Bacchus)a wild gathering involving excessive drinking and promiscuity
bacchant
someone who engages in drinking boutsa drunken reveller; a devotee of Bacchus(classical mythology) a priest or votary of Bacchus
bacciferous
producing or bearing berries
baccivorous
feeding on berries
baccy
leaves of the tobacco plant dried and prepared for smoking or ingestion
bach
lead a bachelor’s existence
bachelor
lead a bachelor’s existence
bachelor girl
a young unmarried woman who lives alone
bachelor’s degree
an academic degree conferred on someone who has successfully completed undergraduate studies
bacillary
relating to or produced by or containing bacilliformed like a bacillus
bacillus
aerobic rod-shaped spore-producing bacterium; often occurring in chainlike formations; found primarily in soil
bacitracin
a polypeptide antibiotic of known chemical structure effective against several types of Gram-positive organisms; usually applied locally
back
related to or located at the back; “the back yard”; “the back entrance”located at or near the back of an animal; “back (or hind) legs”; “the hinder part of a carcass”of an earlier date; “back issues of the magazine”
back country
a remote and undeveloped area
back door
a secret or underhand means of access (to a place or a position); “he got his job through the back door”an undocumented way to get access to a computer system or the data it containsan entrance at the rear of a building
back down
move backwards from a certain position; “The bully had to back down”remove oneself from an obligation; “He bowed out when he heard how much work was involved”
back matter
written matter following the main text of a book
back out
move out of a space backwards; “He backed out of the driveway”make a retreat from an earlier commitment or activity; “We’ll have to crawfish out from meeting with him”; “He backed out of his earlier promise”; “The aggressive investment company pulled in its horns”
back talk
an impudent or insolent rejoinder; “don’t give me any of your sass”
back up
give moral or psychological support
backache
an ache localized in the back
backbencher
a member of the House of Commons who is not a party leader
backbend
an acrobatic feat in which the trunk is bent backward from a standing position until the hands touch the floor
backbite
say mean things
backboard
a raised vertical board with basket attached; used to play basketball; “he banked the shot off the backboard”a board used to support the back of someone or something
backbone
a central cohesive source of support and stability; “faith is his anchor”; “the keystone of campaign reform was the ban on soft money”; “he is the linchpin of this firm”fortitude and determination; “he didn’t have the guts to try it”the series of vertebrae forming the axis of the skeleton and protecting the spinal cord; “the fall broke his back”the part of a book’s cover that encloses the inner side of the book’s pages and that faces outward when the book is shelved; “the title and author were printed on the spine of the book”the part of a network that connects other networks together; “the backbone is the part of a communication network that carries the heaviest traffic”
backbreaking
characterized by effort to the point of exhaustion; especially physical effort; “worked their arduous way up the mining valley”; “a grueling campaign”; “hard labor”; “heavy work”; “heavy going”; “spent many laborious hours on the project”; “set a punishing pace”
backchat
light teasing repartee
backcross
mate a hybrid of the first generation with one of its parents
backdate
make effective from an earlier date; “The increase in tax was backdated to January”
backdrop
scenery hung at back of stage
backed
having a back or backing
backer
invests in a theatrical production
backfield
the offensive football players who line up behind the linemen
backfire
come back to the originator of an action with an undesired effect; “Your comments may backfire and cause you a lot of trouble”emit a loud noise as a result of undergoing a backfire; “My old car backfires all the time”set a controlled fire to halt an advancing forest to prairie fire
backflow
a flow that returns toward its source
backgammon
a board game for two players; pieces move according to throws of the dice
background
understate the importance or quality of; “he played down his royal ancestry”
backhand
(of racket strokes) made across the body with back of hand facing direction of stroke(of handwriting) having the letters slanting backward
backhanded
(of racket strokes) made across the body with back of hand facing direction of strokeroundabout or ambiguous; “attacks from that source amounted to a backhanded compliment to his integrity”; “a backhanded and dishonest way of reaching his goal”
backhander
a backhanded blow
backing
the act of providing approval and support; “his vigorous backing of the conservatives got him in trouble with progressives”something forming a back that is added for strengtheningfinancial resources provided to make some project possible; “the foundation provided support for the experiment”
backlash
come back to the originator of an action with an undesired effect; “Your comments may backfire and cause you a lot of trouble”
backlog
accumulate and create a backlog
backpack
hike with a backpack; “Every summer they are backpacking in the Rockies”
backplate
plate armor protecting the back; worn as part of a cuirass
backrest
a support that you can lean against while sitting; “the back of the dental chair was adjustable”
backsaw
a handsaw that is stiffened by metal reinforcement along the upper edge
backscratcher
someone who is willing to trade favors or services for mutual advantagea long-handled scratcher for scratching your back
backsheesh
a relatively small amount of money given for services rendered (as by a waiter)
backside
the side of an object that is opposite its front; “his room was toward the rear of the hotel”the fleshy part of the human body that you sit on; “he deserves a good kick in the butt”; “are you going to sit on your fanny and do nothing?”
backslide
drop to a lower level
backspace
hit the backspace key on a computer or typewriter keyboard; “To erase
backspin
spin (usually of a moving ball) that retards or reverses the forward motion
backstage
concealed from public view or attention; “offstage political meetings”
backstairs
secret and sly or sordid; “backstairs gossip”; “his low backstairs cunning”- A.L.Guerard; “backstairs intimacies”; “furtive behavior”
backstay
a stay that supports the back of something
backstitch
do backstitches
backstop
act as a backstop
backstroke
swim on one’s back
backswept
used of hair
backsword
a stick used instead of a sword for fencinga sword with only one cutting edge
backtrack
retrace one’s course; “The hikers got into a storm and had to turn back”
backup
an accumulation caused by clogging or a stoppage; “a traffic backup on the main street”; “he discovered a backup in the toilet”someone who takes the place of another (as when things get dangerous or difficult); “the star had a stand-in for dangerous scenes”; “we need extra employees for summer fill-ins”a musical part (vocal or instrumental) that supports or provides background for other musical parts(computer science) a copy of a file or directory on a separate storage device; “he made a backup in case the original was accidentally damaged or erased”the act of providing approval and support; “his vigorous backing of the conservatives got him in trouble with progressives”
backward
directed or facing toward the back or rear; “a backward view”(used of temperament or behavior) marked by a retiring nature; “a backward lover”retarded in intellectual developmenthaving made less than normal progress; “an economically backward country”
backwards
at or to or toward the back or rear; “he moved back”; “tripped when he stepped backward”; “she looked rearward out the window of the car”in a manner or order or direction the reverse of normal; “it’s easy to get the i' and the
e’ backward in words like seize' and
siege’”; “the child put her jersey on backward”
backwash
the flow of air that is driven backwards by an aircraft propellerthe consequences of an event (especially a catastrophic event); “the aftermath of war”; “in the wake of the accident no one knew how many had been injured”the wave that spreads behind a boat as it moves forward; “the motorboat’s wake capsized the canoe”
backwater
a body of water that was created by a flood or tide or by being held or forced back by a dam; “the bayous and backwaters are breeding grounds for mosquitos”a place or condition in which no development or progress is occurring; “the country is an economic backwater”
backwoods
a remote and undeveloped area
backwoodsman
a man who lives on the frontier
bacon
back and sides of a hog salted and dried or smoked; usually sliced thin and friedEnglish scientist and Franciscan monk who stressed the importance of experimentation; first showed that air is required for combustion and first used lenses to correct vision (1220-1292)English statesman and philosopher; precursor of British empiricism; advocated inductive reasoning (1561-1626)
bacteria
(microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered to be plants
bactericide
any chemical agent that destroys bacteriaany drug that destroys bacteria or inhibits their growth
bacteriology
the branch of medical science that studies bacteria in relation to disease
bacteriolysis
dissolution or destruction of bacteria
bacteriophage
a virus that is parasitic (reproduces itself) in bacteria; “phage uses the bacterium’s machinery and energy to produce more phage until the bacterium is destroyed and phage is released to invade surrounding bacteria”
bacteriostasis
inhibition of the growth of bacteria
bacteriostat
a chemical or biological material that inhibits bacterial growth
bacterium
(microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered to be plants
bacteroid
resembling bacteria
baculiform
formed like a bacillus
bad
having undesirable or negative qualities; “a bad report card”; “his sloppy appearance made a bad impression”; “a bad little boy”; “clothes in bad shape”; “a bad cut”; “bad luck”; “the news was very bad”; “the reviews were bad”; “the pay is bad”; “it was a bad light for reading”; “the movie was a bad choice”very intense; “a bad headache”; “in a big rage”; “had a big (or bad) shock”; “a bad earthquake”; “a bad storm”feeling physical discomfort or pain (tough' is occasionally used colloquially for
bad’); “my throat feels bad”; “she felt bad all over”; “he was feeling tough after a restless night”(of foodstuffs) not in an edible or usable condition; “bad meat”; “a refrigerator full of spoilt food”feeling or expressing regret or sorrow or a sense of loss over something done or undone; “felt regretful over his vanished youth”; “regretful over mistakes she had made”; “he felt bad about breaking the vase”not capable of being collected; “a bad (or uncollectible) debt”below average in quality or performance; “a bad chess player”; “a bad recital”nonstandard; “so-called bad grammar”not financially safe or secure; “a bad investment”; “high risk investments”; “anything that promises to pay too much can’t help being risky”; “speculative business enterprises”physically unsound or diseased; “has a bad back”; “a bad heart”; “bad teeth”; “an unsound limb”; “unsound teeth”capable of harming; “bad air”; “smoking is bad for you”characterized by wickedness or immorality; “led a very bad life”reproduced fraudulently; “like a bad penny…”; “a forged twenty dollar bill”not working properly; “a bad telephone connection”; “a defective appliance”
bad blood
a feeling of ill will arousing active hostility
bad check
a check that is dishonored on presentation because of insufficient funds; “issuing a bad check is a form of larceny”
bad debt
a debt that is unlikely to be repaid
bad-tempered
annoyed and irritable
baddie
the principal bad character in a film or work of fiction
bade
a Chadic language spoken in northern Nigeria
badge
put a badge on; “The workers here must be badged”
badger
annoy persistently; “The children teased the boy because of his stammer”persuade through constant efforts
badinage
frivolous banter
badlands
deeply eroded barren landan eroded and barren region in southwestern South Dakota and northwestern Nebraska
badly
to a severe or serious degree; “fingers so badly frozen they had to be amputated”; “badly injured”; “a severely impaired heart”; “is gravely ill”; “was seriously ill”(ill' is often used as a combining form) in a poor or improper or unsatisfactory manner; not well; "he was ill prepared"; "it ill befits a man to betray old friends"; "the car runs badly"; "he performed badly on the exam"; "the team played poorly"; "ill-fitting clothes"; "an ill-conceived plan"evilly or wickedly; "treated his parents badly"; "to steal is to act badly"in a disobedient or naughty way; "he behaved badly in school"; "he mischievously looked for a chance to embarrass his sister"; "behaved naughtily when they had guests and was sent to his room"with great intensity (
bad’ is a nonstandard variant for `badly’); “the injury hurt badly”; “the buildings were badly shaken”; “it hurts bad”; “we need water bad”very much; strongly; “I wanted it badly enough to work hard for it”; “the cables had sagged badly”; “they were badly in need of help”; “he wants a bicycle so bad he can taste it”without skill or in a displeasing manner; “she writes badly”; “I think he paints very badly”in a disadvantageous way; to someone’s disadvantage; “the venture turned out badly for the investors”; “angry that the case was settled disadvantageously for them”unfavorably or with disapproval; “tried not to speak ill of the dead”; “thought badly of him for his lack of concern”with unusual distress or resentment or regret or emotional display; “they took their defeat badly”; “took her father’s death badly”; “conducted himself very badly at the time of the earthquake”
badminton
a game played on a court with light long-handled rackets used to volley a shuttlecock over a net
baffle
be a mystery or bewildering to; “This beats me!”; “Got me–I don’t know the answer!”; “a vexing problem”; “This question really stuck me”hinder or prevent (the efforts
bag
capture or kill
bagasse
the dry dusty pulp that remains after juice is extracted from sugar cane or similar plants
bagatelle
a light piece of music for pianosomething of little value or significancea table game in which short cues are used to knock balls into holes that are guarded by wooden pegs; penalties are incurred if the pegs are knocked over
bagel
(Yiddish) glazed yeast-raised doughnut-shaped roll with hard crust
baggage
cases used to carry belongings when travelinga worthless or immoral womanthe portable equipment and supplies of an army
baggage car
a railway car where passengers’ bags are carried
bagging
coarse fabric used for bags or sacks
baggy
not fitting closely; hanging loosely; “baggy trousers”; “a loose-fitting blouse is comfortable in hot weather”
bagman
a salesman who travels to call on customersa racketeer assigned to collect or distribute payoff money
bagnio
a building where prostitutes are availablea building containing public baths
bagpipe
a tubular wind instrument; the player blows air into a bag and squeezes it out through the drone