From AMS Glossary
Two different imaginary 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 dichroism is elliptical, specific examples of which are linear and circular. The dichroism of a medium originates from its asymmetry.