| once | (adverb) on one occasion | Synonyms: in one case, one time |
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| (adverb) at a previous time | Synonyms: at one time, erst, erstwhile, formerly |
| (adverb) as soon as | - |
| 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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| oncologic | (adjective) of or relating to or practicing oncology | Synonyms: oncological |
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| oncological | (adjective) of or relating to or practicing oncology | Synonyms: oncologic |
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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 | (adjective) moving toward one | - |
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| (noun) the beginning or early stages | Synonyms: onset |