| octad | (noun) the cardinal number that is the sum of seven and one | Synonyms: 8, eight, eighter, eighter from Decatur, octet, octonary, ogdoad, VIII |
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| octagon | (noun) an eight-sided polygon | - |
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| octagonal | (adjective) of or relating to or shaped like an octagon | Synonyms: octangular |
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| octahedron | (noun) any polyhedron having eight plane faces | - |
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| octal | (adjective) of or pertaining to a number system having 8 as its base | - |
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| octameter | (noun) a verse line having eight metrical feet | - |
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| octane | (noun) any isomeric saturated hydrocarbon found in petroleum and used as a fuel and solvent | - |
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| octangular | (adjective) of or relating to or shaped like an octagon | Synonyms: octagonal |
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| octant | (noun) a measuring instrument for measuring angles to a celestial body; similar to a sextant but with 45 degree calibration | - |
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| octave | (noun) a musical interval of eight tones | Synonyms: musical octave |
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| (noun) a rhythmic group of eight lines of verse | - |
| (noun) a feast day and the seven days following it | - |
| octavo | (noun) the size of a book whose pages are made by folding a sheet of paper three times to form eight leaves | Synonyms: 8vo, eightvo |
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| octet | (noun) a musical composition written for eight performers | Synonyms: octette |
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| (noun) eight people considered as a unit | Synonyms: eightsome, octette |
| (noun) a set of eight similar things considered as a unit | Synonyms: octette |
| (noun) eight performers or singers who perform together | Synonyms: octette |
| (noun) the cardinal number that is the sum of seven and one | Synonyms: 8, eight, eighter, eighter from Decatur, octad, octonary, ogdoad, VIII |
| octette | (noun) a musical composition written for eight performers | Synonyms: octet |
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| (noun) eight people considered as a unit | Synonyms: eightsome, octet |
| (noun) a set of eight similar things considered as a unit | Synonyms: octet |
| (noun) eight performers or singers who perform together | Synonyms: octet |
| octillion | (noun) the number that is represented as a one followed by 27 zeros | - |
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| octogenarian | (adjective) being from 80 to 89 years old | - |
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| (noun) someone whose age is in the eighties | - |
| octonary | (noun) the cardinal number that is the sum of seven and one | Synonyms: 8, eight, eighter, eighter from Decatur, octad, octet, ogdoad, VIII |
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| octopod | (noun) a cephalopod with eight arms but lacking an internal shell | - |
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| octopus | (noun) bottom-living cephalopod having a soft oval body with eight long tentacles | Synonyms: devilfish |
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| (noun) tentacles of octopus prepared as food | - |
| octoroon | (noun) an offspring of a quadroon and a white parent; a person who is one-eighth black | - |
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| octosyllabic | (adjective) having or characterized by or consisting of eight syllables | - |
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| octosyllable | (noun) a verse line having eight syllables or a poem of octosyllabic lines | - |
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| octroi | (noun) a tax on various goods brought into a town | - |
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| octuple | (adjective) having eight units or components | Synonyms: eight-fold, eightfold |
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