'optic axis' definitions:

In 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"

A 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" [syn: principal axis, optic axis]