optic axis: Meaning and Definition of

op'tic ax'is

Pronunciation: [key]
— Crystall. Crystall.
  1. (in a crystal exhibiting double refraction) the direction or directions, uniaxial or biaxial, respectively, along which this phenomenon does not occur.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.