Definitions for optic axis
Random House Webster's College Dictionary
the direction or directions in a crystal along which light is not doubly refracted.
Origin of optic axis:
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"
principal axis, optic axis(noun)
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"