optic axis (Noun) — In a doubly refracting crystal, the line in the direction of which no double refraction occurs. ex. "a crystal may have either one or two optic axes"
optic axis (Noun) — A line that passes through the centre of curvature of a lens so that light is neither reflected nor refracted. ex. "in a normal eye the optic axis is the direction in which objects are seen most distinctly"