Etymology
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Words related to octa-

eight (adj., n.)

"1 more than seven, twice four; the number which is one more than seven; a symbol representing this number;" late 14c., eighte, earlier ehte (c. 1200), from Old English eahta, æhta, from Proto-Germanic *akhto (source also of Old Saxon ahto, Old Frisian ahta, Old Norse atta, Swedish åtta, Dutch acht, Old High German Ahto, German acht, Gothic ahtau), from PIE *okto(u) "eight" (source also of Sanskrit astau, Avestan ashta, Greek okto, Latin octo, Old Irish ocht-n, Breton eiz, Old Church Slavonic osmi, Lithuanian aštuoni). From the Latin word come Italian otto, Spanish ocho, Old French oit, Modern French huit. For spelling, see fight (v.).

Meaning "eight-man crew of a rowing boat" is from 1847. The Spanish piece of eight (1690s) was so called because it was worth eight reals (see piece (n.)). Figure (of) eight as the shape of a race course, etc., attested from c. 1600. To be behind the eight ball "in trouble" (1932) is a metaphor from shooting pool. Eight hours as the ideal length of a fair working day is recorded by 1845.

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octad (n.)

"a group, system, or series of eight," 1801; see octa- + -ad.

octahedron (n.)

"a solid figure bounded by eight plane faces," 1560s, from Greek oktahedron, neuter of oktahedros "eight-sided," from okta- "eight" (see octa-) + hedra "a seat; face of a geometrical solid," from PIE root *sed- (1) "to sit." Related: Octahedral.

octane (n.)

hydrocarbon of the methane series, 1872, coined from oct- "eight" (see octa-) + -ane; so called because it has eight carbon atoms. A fuel's octane rating, in reference to its anti-knocking quality, is attested from 1932.

octant (n.)

instrument for making angular measurements in navigation or astronomy, 1731, from Late Latin octans "the eighth part," from octo "eight" (see octa-) on analogy of quadrant. In geometry, "the eighth part of a circle," by 1750.

octo- 

word-forming element, before vowels oct-, from combining form of Latin octo "eight," from PIE root *octo(u)- "eight" (see eight). Words made from Greek elements or derived from Greek typically are octa-.