| omasum | (noun) the third compartment of the stomach of a ruminant | Synonyms: psalterium, third stomach |
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| ombu | (noun) fast-growing herbaceous evergreen tree of South America having a broad trunk with high water content and dark green oval leaves | Synonyms: bella sombra, Phytolacca dioica |
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| ombudsman | (noun) a government appointee who investigates complaints by private persons against the government | - |
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| omega | (noun) the last (24th) letter of the Greek alphabet | - |
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| (noun) the ending of a series or sequence | Synonyms: Z |
| omelet | (noun) beaten eggs or an egg mixture cooked until just set; may be folded around e.g. ham or cheese or jelly | Synonyms: omelette |
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| omelette | (noun) beaten eggs or an egg mixture cooked until just set; may be folded around e.g. ham or cheese or jelly | Synonyms: omelet |
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| omen | (noun) a sign of something about to happen | Synonyms: portent, presage, prodigy, prognostic, prognostication |
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| omentum | (noun) a fold of peritoneum supporting the viscera | - |
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| omeprazole | (noun) antacid (trade name Prilosec) that suppresses acid secretion in the stomach | Synonyms: Prilosec |
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| omerta | (noun) a code of silence practiced by the Mafia; a refusal to give evidence to the police about criminal activities | - |
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| omicron | (noun) the 15th letter of the Greek alphabet | - |
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| ominousness | (noun) The quality of giving indication of a coming ill; being an evil omen; being threatening, portentous, inauspicious. | - |
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| omission | (noun) a mistake resulting from neglect | Synonyms: skip |
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| (noun) neglecting to do something; leaving out or passing over something | - |
| (noun) any process whereby sounds or words are left out of spoken words or phrases | Synonyms: deletion |
| (noun) something that has been omitted | - |
| ommastrephes | (noun) extremely active cylindrical squid with short strong arms and large rhombic terminal fins | - |
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| ommatidium | (noun) any of the numerous small cone-shaped eyes that make up the compound eyes of some arthropods | - |
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| omnibus | (noun) a vehicle carrying many passengers; used for public transport | Synonyms: autobus, bus, charabanc, coach, double-decker, jitney, motorbus, motorcoach, passenger vehicle |
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| (noun) an anthology of articles on a related subject or an anthology of the works of a single author | - |
| omnipotence | (noun) the state of being omnipotent; having unlimited power | - |
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| omnipresence | (noun) the state of being everywhere at once (or seeming to be everywhere at once) | Synonyms: ubiquitousness, ubiquity |
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| omnirange | (noun) a navigational system consisting of a network of radio beacons that provide aircraft with information about exact position and bearing | Synonyms: omnidirectional radio range, omnidirectional range |
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| omniscience | (noun) the state of being omniscient; having infinite knowledge | - |
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| omnivore | (noun) an animal that feeds on both animal and vegetable substances | - |
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| (noun) a person who eats all kinds of foods | - |
| omnivorousness | (noun) The state of eating both animal and plant matter. | - |
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| omophagia | (noun) the eating of raw food | - |
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| omphalocele | (noun) protrusion of the intestine and omentum through a hernia in the abdominal wall near the navel; usually self correcting after birth | Synonyms: umbilical hernia |
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| omphalos | (noun) a scar where the umbilical cord was attached | Synonyms: belly button, bellybutton, navel, omphalus, umbilicus |
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| omphaloskepsis | (noun) literally, the contemplation of one's navel, which is an idiom usually meaning complacent self-absorption | Synonyms: navel-gazing |
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| omphalus | (noun) a scar where the umbilical cord was attached | Synonyms: belly button, bellybutton, navel, omphalos, umbilicus |
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| onager | (noun) Asiatic wild ass | Synonyms: Equus hemionus |
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| (noun) small catapult used by the Romans | - |
| onanism | (noun) manual stimulation of the genital organs (of yourself or another) for sexual pleasure | Synonyms: masturbation |
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| (noun) a method of birth control in which coitus is initiated but the penis is deliberately withdrawn before ejaculation | Synonyms: coitus interruptus, pulling out, withdrawal, withdrawal method |
| onanist | (noun) a person who practices masturbation | Synonyms: masturbator |
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| onchocerciasis | (noun) infestation with slender threadlike roundworms (filaria) deposited under the skin by the bite of black fleas; when the eyes are involved it can result in blindness; common in Africa and tropical America | Synonyms: river blindness |
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| oncidium | (noun) any orchid of the genus Oncidium: characterized by slender branching sprays of small yellow and brown flowers; often grown as houseplants | Synonyms: butterfly orchid, butterfly plant, dancing lady orchid |
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| oncogene | (noun) a gene that disposes normal cells to change into cancerous tumor cells | Synonyms: transforming gene |
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| oncologist | (noun) a specialist in oncology | - |
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| oncology | (noun) the branch of medicine concerned with the study and treatment of tumors | - |
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| oncoming | (noun) the beginning or early stages | Synonyms: onset |
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| one | (noun) a single person or thing | - |
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| (noun) the smallest whole number or a numeral representing this number | Synonyms: 1, ace, I, single, unity |
| oneirism | (noun) absentminded dreaming while awake | Synonyms: air castle, castle in Spain, castle in the air, daydream, daydreaming, reverie, revery |
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| oneiromancer | (noun) someone who divines through the interpretation of dreams | - |
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| oneiromancy | (noun) divination through the interpretation of dreams | - |
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| oneness | (noun) the quality of being united into one | Synonyms: unity |
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| onerousness | (noun) unwelcome burdensome difficulty | Synonyms: burdensomeness, heaviness, oppressiveness |
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| onion | (noun) an aromatic flavorful vegetable | - |
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| (noun) bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb | Synonyms: Allium cepa, onion plant |
| (noun) the bulb of an onion plant | - |
| onionskin | (noun) a thin strong lightweight translucent paper used especially for making carbon copies | Synonyms: flimsy |
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| onkos | (noun) A high ornate headdress worn by the performers of Greek tragedy. | - |
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| onlooker | (noun) someone who looks on | Synonyms: looker-on |
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| onomancer | (noun) one who practices onomancy | - |
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| onomancy | (noun) divination by the letters of a name | - |
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| onomasticon | (noun) a list of proper nouns naming persons or places | - |
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| onomastics | (noun) the branch of lexicology that studies the forms and origins of proper names | - |
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| onomatomania | (noun) obsession with a particular word which the person uses repeatedly or which intrudes into consciousness | - |
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| onomatopoeia | (noun) using words that imitate the sound they denote | - |
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| onrush | (noun) (military) an offensive against an enemy (using weapons) | Synonyms: attack, onset, onslaught |
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| (noun) a forceful forward rush or flow | - |
| onset | (noun) (military) an offensive against an enemy (using weapons) | Synonyms: attack, onrush, onslaught |
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| (noun) the beginning or early stages | Synonyms: oncoming |
| onslaught | (noun) (military) an offensive against an enemy (using weapons) | Synonyms: attack, onrush, onset |
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| (noun) the rapid and continuous delivery of linguistic communication (spoken or written) | Synonyms: barrage, bombardment, outpouring |
| (noun) a sudden and severe onset of trouble | - |
| ontogenesis | (noun) (biology) the process of an individual organism growing organically; a purely biological unfolding of events involved in an organism changing gradually from a simple to a more complex level | Synonyms: development, growing, growth, maturation, ontogeny |
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| ontogeny | (noun) (biology) the process of an individual organism growing organically; a purely biological unfolding of events involved in an organism changing gradually from a simple to a more complex level | Synonyms: development, growing, growth, maturation, ontogenesis |
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| ontology | (noun) the metaphysical study of the nature of being and existence | - |
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| (noun) (computer science) a rigorous and exhaustive organization of some knowledge domain that is usually hierarchical and contains all the relevant entities and their relations | - |
| onus | (noun) an onerous or difficult concern | Synonyms: burden, encumbrance, incumbrance, load |
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| onycholysis | (noun) separation of a nail from its normal attachment to the nail bed | - |
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| onychophoran | (noun) any of numerous velvety-skinned wormlike carnivorous animals common in tropical forests having characteristics of both arthropods and annelid worms | Synonyms: peripatus, velvet worm |
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| onychosis | (noun) any disease or disorder of the nails | - |
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| onyx | (noun) a chalcedony with alternating black and white bands; used in making cameos | - |
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| onyxis | (noun) toenail having its free tip or edges embedded in the surrounding flesh | Synonyms: ingrown toenail |
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| oocyte | (noun) a female gametocyte that develops into an ovum after two meiotic divisions | - |
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| oodles | (noun) a large number or amount | Synonyms: dozens, gobs, heaps, lashings, loads, lots, piles, rafts, scads, scores, slews, stacks, tons, wads |
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| oogenesis | (noun) development of ova | - |
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| oology | (noun) the branch of zoology that studies eggs (especially birds' eggs and their size, shape, coloration, and number) | - |
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| oolong | (noun) Chinese tea leaves that have been partially fermented before being dried | - |
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| oomph | (noun) the activeness of an energetic personality | Synonyms: dynamism, pizzaz, pizzazz, zing |
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| (noun) attractiveness to the opposite sex | Synonyms: desirability, desirableness, sex appeal |
| oophorectomy | (noun) surgical removal of one or both ovaries | Synonyms: ovariectomy |
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| oophoritis | (noun) inflammation of one or both ovaries | - |
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| oophorosalpingectomy | (noun) surgical removal of one or both ovaries and the corresponding Fallopian tubes | - |
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| oosphere | (noun) a gamete; used especially of lower plants | - |
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| oospore | (noun) a thick-walled sexual spore that develops from a fertilized oosphere in some algae and fungi | - |
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| ootid | (noun) mature ovum after penetration by sperm but before the formation of a zygote | - |
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| ooze | (noun) the process of seeping | Synonyms: oozing, seepage |
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| (noun) any thick, viscous matter | Synonyms: goo, gook, goop, guck, gunk, muck, slime, sludge |
| oozing | (noun) the process of seeping | Synonyms: ooze, seepage |
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| opacification | (noun) the process of becoming cloudy or opaque | - |
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| opacity | (noun) the quality of being opaque to a degree; the degree to which something reduces the passage of light | Synonyms: opaqueness |
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| (noun) incomprehensibility resulting from obscurity of meaning | Synonyms: opaqueness |
| (noun) the phenomenon of not permitting the passage of electromagnetic radiation | - |
| opah | (noun) large elliptical brightly colored deep-sea fish of Atlantic and Pacific and Mediterranean | Synonyms: Lampris regius, moonfish |
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| opal | (noun) a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones | - |
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| opalescence | (noun) the visual property of something having a milky brightness and a play of colors from the surface | Synonyms: iridescence |
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| opaqueness | (noun) the quality of being opaque to a degree; the degree to which something reduces the passage of light | Synonyms: opacity |
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| (noun) incomprehensibility resulting from obscurity of meaning | Synonyms: opacity |
| open | (noun) information that has become public | Synonyms: surface |
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| (noun) a tournament in which both professionals and amateurs may play | - |
| (noun) a clear or unobstructed space or expanse of land or water | Synonyms: clear |
| (noun) where the air is unconfined | Synonyms: open air, out-of-doors, outdoors |
| openbill | (noun) stork with a grooved bill whose upper and lower parts touch only at the base and tip | - |
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| opener | (noun) a hand tool used for opening sealed containers (bottles or cans) | - |
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| (noun) the first event in a series | - |
| (noun) a person who unfastens or unwraps or opens | Synonyms: undoer, unfastener, untier |
| openhandedness | (noun) liberality in bestowing gifts; extremely liberal and generous of spirit | Synonyms: largess, largesse, magnanimity, munificence |
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| opening | (noun) a recognized sequence of moves at the beginning of a game of chess | Synonyms: chess opening |
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| (noun) the act of opening something | - |
| (noun) becoming open or being made open | - |
| (noun) the first of a series of actions | Synonyms: first step, initiative, opening move |
| (noun) a vacant or unobstructed space that is man-made | - |
| (noun) an entrance equipped with a hatch; especially a passageway between decks of a ship | Synonyms: hatchway, scuttle |
| (noun) an aperture or hole that opens into a bodily cavity | Synonyms: orifice, porta |
| (noun) a possible alternative | Synonyms: possibility, possible action |
| (noun) the initial part of the introduction | - |
| (noun) the first performance (as of a theatrical production) | Synonyms: curtain raising, opening night |
| (noun) a ceremony accompanying the start of some enterprise | - |
| (noun) an open or empty space in or between things | Synonyms: gap |
| (noun) opportunity especially for employment or promotion | - |
| openness | (noun) characterized by an attitude of ready accessibility (especially about one's actions or purposes); without concealment; not secretive | Synonyms: nakedness |
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| (noun) willingness or readiness to receive (especially impressions or ideas) | Synonyms: receptiveness, receptivity |
| (noun) without obstructions to passage or view | - |
| openwork | (noun) ornamental work (such as embroidery or latticework) having a pattern of openings | - |
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| opepe | (noun) large African forest tree yielding a strong hard yellow to golden brown lumber; sometimes placed in genus Sarcocephalus | Synonyms: Nauclea diderrichii, Sarcocephalus diderrichii |
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| opera | (noun) a building where musical dramas are performed | Synonyms: opera house |
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| (noun) a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes | - |
| operagoer | (noun) a patron of the opera | - |
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| operand | (noun) a quantity upon which a mathematical operation is performed | - |
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| operation | (noun) a medical procedure involving an incision with instruments; performed to repair damage or arrest disease in a living body | Synonyms: surgery, surgical operation, surgical procedure, surgical process |
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| (noun) (mathematics) calculation by mathematical methods | Synonyms: mathematical operation, mathematical process |
| (noun) activity by a military or naval force (as a maneuver or campaign) | Synonyms: military operation |
| (noun) a planned activity involving many people performing various actions | - |
| (noun) a process or series of acts especially of a practical or mechanical nature involved in a particular form of work | Synonyms: procedure |
| (noun) a business especially one run on a large scale | - |
| (noun) the activity of operating something (a machine or business etc.) | - |
| (noun) (psychology) the performance of some composite cognitive activity; an operation that affects mental contents | Synonyms: cognitive operation, cognitive process, mental process, process |
| (noun) (computer science) data processing in which the result is completely specified by a rule (especially the processing that results from a single instruction) | - |
| (noun) process or manner of functioning or operating | Synonyms: functioning, performance |
| (noun) the state of being in effect or being operative | - |
| operationalism | (noun) (philosophy) the doctrine that the meaning of a proposition consists of the operations involved in proving or applying it | - |
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| operations | (noun) financial transactions at a brokerage; having to do with the execution of trades and keeping customer records | Synonyms: trading operations |
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| operative | (noun) someone who can be employed as a detective to collect information and assist in criminal investigations | Synonyms: PI, private detective, private eye, private investigator, shamus, sherlock |
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| (noun) a person secretly employed in espionage for a government | Synonyms: intelligence agent, intelligence officer, secret agent |
| operator | (noun) an agent that operates some apparatus or machine | Synonyms: manipulator |
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| (noun) someone who owns or operates a business | - |
| (noun) a shrewd or unscrupulous person who knows how to circumvent difficulties | Synonyms: hustler, wheeler dealer |
| (noun) a speculator who trades aggressively on stock or commodity markets | - |
| (noun) (mathematics) a symbol or function representing a mathematical operation | - |