optic axis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An optical path through a crystal along which a ray of light can pass without undergoing double refraction.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted
• noun in a doubly refracting crystal, the line in the direction of which no double refraction occurs