| backstairs | (adjective) secret and sly or sordid | Synonyms: backstair, furtive |
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| (noun) a second staircase at the rear of a building | - |
| backstay | (noun) a stay that supports the back of something | - |
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| backstitch | (noun) an overlapping stitch made by starting the next stitch at the middle of the preceding one | - |
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| (verb) do backstitches | - |
| backstop | (noun) a precaution in case of an emergency | - |
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| (noun) (baseball) a fence or screen (as behind home plate) to prevent the ball from traveling out of the playing field | - |
| (noun) (baseball) the person who plays the position of catcher | Synonyms: catcher |
| (verb) act as a backstop | - |
| backstroke | (noun) a swimming stroke that resembles the crawl except the swimmer lies on his or her back | - |
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| (verb) swim on one's back | - |
| backstroker | (noun) someone who swims the backstroke | - |
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| backswept | (adjective) used of hair | Synonyms: sweptback |
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| backswimmer | (noun) predaceous aquatic insect that swims on its back and may inflict painful bites | Synonyms: Notonecta undulata |
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| backsword | (noun) a sword with only one cutting edge | - |
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| (noun) a stick used instead of a sword for fencing | Synonyms: fencing stick, singlestick |
| backtalk | (noun) an impudent or insolent rejoinder | Synonyms: back talk, lip, mouth, sass, sassing |
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| backtrack | (verb) retrace one's course | Synonyms: double back, turn back |
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| backup | (noun) the act of providing approval and support | Synonyms: backing, championship, patronage |
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| (noun) (computer science) a copy of a file or directory on a separate storage device | Synonyms: computer backup |
| (noun) a musical part (vocal or instrumental) that supports or provides background for other musical parts | Synonyms: accompaniment, musical accompaniment, support |
| (noun) someone who takes the place of another (as when things get dangerous or difficult) | Synonyms: backup man, fill-in, relief, reliever, stand-in, substitute |
| (noun) an accumulation caused by clogging or a stoppage | - |
| backward | (adjective) directed or facing toward the back or rear | - |
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| (adjective) (used of temperament or behavior) marked by a retiring nature | - |
| (adjective) retarded in intellectual development | Synonyms: feebleminded, half-witted, slow-witted |
| (adjective) having made less than normal progress | - |
| (adverb) in a manner or order or direction the reverse of normal | Synonyms: backwards |
| (adverb) in or to or toward a past time | Synonyms: back |
| (adverb) at or to or toward the back or rear | Synonyms: back, backwards, rearward, rearwards |
| backwardation | (noun) the market condition wherein the price of a forward or futures contract is trading below the expected spot price at contract maturity. | - |
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| backwardness | (noun) lack of normal development of intellectual capacities | Synonyms: mental retardation, retardation, slowness, subnormality |
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| backwards | (adverb) in a manner or order or direction the reverse of normal | Synonyms: backward |
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| (adverb) at or to or toward the back or rear | Synonyms: back, backward, rearward, rearwards |
| backwash | (noun) the wave that spreads behind a boat as it moves forward | Synonyms: wake |
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| (noun) the flow of air that is driven backwards by an aircraft propeller | Synonyms: airstream, race, slipstream, wash |
| (noun) the consequences of an event (especially a catastrophic event) | Synonyms: aftermath, wake |
| backwater | (noun) a place or condition in which no development or progress is occurring | - |
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| (noun) a body of water that was created by a flood or tide or by being held or forced back by a dam | - |
| backwoods | (noun) a remote and undeveloped area | Synonyms: back country, boondocks, hinterland |
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| backwoodsman | (noun) a man who lives on the frontier | Synonyms: frontiersman, mountain man |
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| backyard | (noun) the grounds in back of a house | - |
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| bacon | (noun) back and sides of a hog salted and dried or smoked; usually sliced thin and fried | - |
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| bacteremia | (noun) transient presence of bacteria (or other microorganisms) in the blood | Synonyms: bacteriaemia, bacteriemia |
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| bacteremic | (adjective) of or relating to or having bacteremia | - |
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| bacteria | (noun) (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 | Synonyms: bacterium |
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| bacteriacide | (noun) any chemical agent that destroys bacteria | Synonyms: bactericide |
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| bacteriaemia | (noun) transient presence of bacteria (or other microorganisms) in the blood | Synonyms: bacteremia, bacteriemia |
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| bacterial | (adjective) relating to or caused by bacteria | - |
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| bacterially | (adverb) by bacteria | - |
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| bactericidal | (adjective) preventing infection by inhibiting the growth or action of microorganisms | Synonyms: disinfectant, germicidal |
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| bactericide | (noun) any drug that destroys bacteria or inhibits their growth | Synonyms: antibacterial, antibacterial drug |
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| (noun) any chemical agent that destroys bacteria | Synonyms: bacteriacide |
| bacteriemia | (noun) transient presence of bacteria (or other microorganisms) in the blood | Synonyms: bacteremia, bacteriaemia |
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| bacteriochlorophyll | (noun) a substance in photosensitive bacteria that is related to but different from chlorophyll of higher plants | - |
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| bacterioid | (adjective) resembling bacteria | Synonyms: bacterioidal, bacteroid, bacteroidal |
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| bacterioidal | (adjective) resembling bacteria | Synonyms: bacterioid, bacteroid, bacteroidal |
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| bacteriologic | (adjective) of or relating to bacteriology | Synonyms: bacteriological |
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| bacteriological | (adjective) of or relating to bacteriology | Synonyms: bacteriologic |
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| bacteriologist | (noun) a biologist who studies bacteria | - |
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| bacteriology | (noun) the branch of medical science that studies bacteria in relation to disease | - |
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| bacteriolysis | (noun) dissolution or destruction of bacteria | - |
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| bacteriolytic | (adjective) of or relating to or causing bacteriolysis | - |
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| bacteriophage | (noun) a virus that is parasitic (reproduces itself) in bacteria | Synonyms: phage |
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| bacteriophagic | (adjective) of or relating to bacteriophages | Synonyms: bacteriophagous |
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| bacteriophagous | (adjective) of or relating to bacteriophages | Synonyms: bacteriophagic |
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| bacteriostasis | (noun) inhibition of the growth of bacteria | - |
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| bacteriostat | (noun) a chemical or biological material that inhibits bacterial growth | - |
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| bacteriostatic | (adjective) of or relating to or causing bacteriostasis | - |
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| bacterise | (verb) subject to the action of bacteria | Synonyms: bacterize |
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| bacterium | (noun) (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 | Synonyms: bacteria |
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| bacterize | (verb) subject to the action of bacteria | Synonyms: bacterise |
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| bacteroid | (adjective) resembling bacteria | Synonyms: bacterioid, bacterioidal, bacteroidal |
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| (noun) a rodlike bacterium (especially any of the rod-shaped or branched bacteria in the root nodules of nitrogen-fixing plants) | - |
| bacteroidal | (adjective) resembling bacteria | Synonyms: bacterioid, bacterioidal, bacteroid |
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| baculiform | (adjective) formed like a bacillus | Synonyms: bacillar, bacillary, bacilliform, rod-shaped |
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| barbacan | (noun) a tower that is part of a defensive structure (such as a castle) | Synonyms: barbican |
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| bareback | (adjective) riding without a saddle | Synonyms: barebacked |
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| (adverb) without a saddle | Synonyms: barebacked |
| barebacked | (adjective) riding without a saddle | Synonyms: bareback |
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| (adverb) without a saddle | Synonyms: bareback |
| biofeedback | (noun) a training program in which a person is given information about physiological processes (heart rate or blood pressure) that is not normally available with the goal of gaining conscious control of them | - |
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| blowback | (noun) Blowback is a system of operation for self-loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gases created by the ignition of the propellant charge. | - |
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| (noun) misinformation resulting from the recirculation into the source country of disinformation previously planted abroad by that country's intelligence service | - |
| (noun) the backward escape of gases and unburned gunpowder after a gun is fired | Synonyms: backfire |
| bulbaceous | (adjective) producing or growing from bulbs | - |
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| buyback | (noun) the act of purchasing back something previously sold | Synonyms: redemption, repurchase |
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| callback | (noun) a request by the manufacturer of a defective product to return the product (as for replacement or repair) | Synonyms: recall |
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| canvasback | (noun) North American wild duck valued for sport and food | Synonyms: Aythya valisineria, canvasback duck |
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| celibacy | (noun) abstaining from sexual relations (as because of religious vows) | Synonyms: chastity, sexual abstention |
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| (noun) an unmarried status | - |
| chawbacon | (noun) a person who is not very intelligent or interested in culture | Synonyms: bumpkin, hayseed, hick, rube, yahoo, yokel |
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| clawback | (noun) finding a way to take money back from people that they were given in another way | - |
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| coccobacillus | (noun) a bacterial cell intermediate in morphology between a coccus and a bacillus; a very short bacillus | - |
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| comeback | (noun) return by a celebrity to some previously successful activity | - |
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| (noun) a quick reply to a question or remark (especially a witty or critical one) | Synonyms: counter, rejoinder, replication, retort, return, riposte |
| cornerback | (noun) a defensive football player stationed outside the linebackers | - |
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| corynebacterium | (noun) any species of the genus Corynebacterium | - |
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| crookback | (adjective) characteristic of or suffering from kyphosis, an abnormality of the vertebral column | Synonyms: crookbacked, gibbous, humpbacked, humped, hunchbacked, kyphotic |
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| (noun) a person whose back is hunched because of abnormal curvature of the upper spine | Synonyms: humpback, hunchback |
| crookbacked | (adjective) characteristic of or suffering from kyphosis, an abnormality of the vertebral column | Synonyms: crookback, gibbous, humpbacked, humped, hunchbacked, kyphotic |
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| cutback | (noun) a reduction in quantity or rate | - |
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| cyanobacteria | (noun) predominantly photosynthetic prokaryotic organisms containing a blue pigment in addition to chlorophyll; occur singly or in colonies in diverse habitats; important as phytoplankton | Synonyms: blue-green algae |
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| cyanobacterial | (adjective) relating to or caused by photosynthetic bacteria of the class Cyanobacteria | Synonyms: cyanophyte |
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| debacle | (noun) a sound defeat | Synonyms: drubbing, slaughter, thrashing, trouncing, walloping, whipping |
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| (noun) a sudden and violent collapse | Synonyms: fiasco |
| (noun) flooding caused by a tumultuous breakup of ice in a river during the spring or summer | - |
| diamondback | (noun) large deadly rattlesnake with diamond-shaped markings | Synonyms: Crotalus adamanteus, diamondback rattlesnake |
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| dieback | (noun) a disease of plants characterized by the gradual dying of the young shoots starting at the tips and progressing to the larger branches | - |
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| drawback | (noun) the quality of being a hindrance | - |
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| enterobacteria | (noun) rod-shaped Gram-negative bacteria; most occur normally or pathogenically in intestines of humans and other animals | Synonyms: enteric bacteria, enterics, entric |
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| eubacteria | (noun) a large group of bacteria having rigid cell walls; motile types have flagella | Synonyms: eubacterium, true bacteria |
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| eubacterium | (noun) a large group of bacteria having rigid cell walls; motile types have flagella | Synonyms: eubacteria, true bacteria |
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| fallback | (noun) to break off a military action with an enemy | Synonyms: disengagement, pullout |
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| fatback | (noun) salt pork from the back of a hog carcass | - |
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| feedback | (noun) response to an inquiry or experiment | - |
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| (noun) the process in which part of the output of a system is returned to its input in order to regulate its further output | - |
| finback | (noun) large flat-headed whalebone whale having deep furrows along the throat; of Atlantic and Pacific | Synonyms: Balaenoptera physalus, common rorqual, fin whale, finback whale |
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| flashback | (noun) a transition (in literary or theatrical works or films) to an earlier event or scene that interrupts the normal chronological development of the story | - |
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| (noun) an unexpected but vivid recurrence of a past experience (especially a recurrence of the effects of an hallucinogenic drug taken much earlier) | - |
| follback | (verb) to return someone's following of you on a social media platform | - |
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| fullback | (noun) (American football) the position of a back on a football team, generally larger in size than halfbacks | - |
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| (noun) (football) the running back who plays the fullback position on the offensive team | - |
| (verb) play the fullback | - |
| grayback | (noun) a dowitcher with a grey back | Synonyms: greyback, Limnodromus griseus |
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| (noun) a sandpiper that breeds in the Arctic and winters in the Southern Hemisphere | Synonyms: Calidris canutus, greyback, knot |
| greenback | (noun) a piece of paper money (especially one issued by a central bank) | Synonyms: bank bill, bank note, banker's bill, banknote, bill, Federal Reserve note, government note, note |
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| greyback | (noun) a dowitcher with a grey back | Synonyms: grayback, Limnodromus griseus |
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| (noun) a sandpiper that breeds in the Arctic and winters in the Southern Hemisphere | Synonyms: Calidris canutus, grayback, knot |
| (noun) `Johnny' was applied as a nickname for Confederate soldiers by the Federal soldiers in the American Civil War; `greyback' derived from their grey Confederate uniforms | Synonyms: Johnny, Johnny Reb, Reb, Rebel |
| halfback | (noun) (American football) the position of a back on a football team, generally smaller in size than a fullback | - |
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| (noun) (football) the running back who plays the offensive halfback position | - |
| halobacter | (noun) halophiles in saline environments such as the Dead Sea or salt flats | Synonyms: halobacteria, halobacterium |
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| halobacteria | (noun) halophiles in saline environments such as the Dead Sea or salt flats | Synonyms: halobacter, halobacterium |
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| halobacterium | (noun) halophiles in saline environments such as the Dead Sea or salt flats | Synonyms: halobacter, halobacteria |
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| hardback | (adjective) having a hard back or cover | Synonyms: hardbacked, hardbound, hardcover |
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| (noun) a book with cardboard or cloth or leather covers | Synonyms: hardcover |
| hardbacked | (adjective) having a hard back or cover | Synonyms: hardback, hardbound, hardcover |
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| hatchback | (noun) a sloping rear car door that is lifted to open | Synonyms: hatch, hatchback door, liftgate |
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| (noun) a car having a hatchback door | - |
| herbaceous | (adjective) characteristic of a nonwoody herb or plant part | - |
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| hibachi | (noun) a portable brazier that burns charcoal and has a grill for cooking | - |
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| (verb) cook over a hibachi grill | - |