From Glossary of Meteorology


(Nearly synonymous with double refraction.) Two different real parts of the refractive index for electromagnetic waves, identical except for their states of (orthogonal) polarization.

By orthogonal is meant that the waves have opposite handedness, the same ellipticity, and the major axes of their vibration ellipses are perpendicular to each other. The most general birefringence is elliptical, specific examples of which are linear and circular. The birefringence of a medium originates from its asymmetry.
See polarization, dichroism;
compare double refraction.