| 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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