| 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| (noun) a method of birth control in which coitus is initiated but the penis is deliberately withdrawn before ejaculation | Synonyms: coitus interruptus, pulling out, withdrawal, withdrawal method |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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