Open English Wordnet
Nouns
(n)optic axisin a doubly refracting crystal, the line in the direction of which no double refraction occurs“a crystal may have either one or two optic axes”
(n)principal axis, optic axisa line that passes through the center of curvature of a lens so that light is neither reflected nor refracted“in a normal eye the optic axis is the direction in which objects are seen most distinctly”