The Collaborative International Dictionary of English v.0.48:

Pleochroism \Ple*och"ro*ism\, n. [Gr.? mor? + ? color.]
   (Crystallog.)
   The property possessed by some crystals, of showing different
   colors when viewed in the direction of different axes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pleochroism
    n 1: the phenomenon of different colors appearing when certain
         crystals are viewed from different directions