Source : Webster's Revised Unabridged Dictionary (1913)

Anorthite \A*nor"thite\, n. [Gr. 'an priv. + ? straight (? sc. ?
   right angle); not in a right angle.]
   A mineral of the feldspar family, commonly occurring in small
   glassy crystals, also a constituent of some igneous rocks. It
   is a lime feldspar. See {Feldspar}.

Source : WordNet®

anorthite
     n : rare plagioclastic feldspar occurring in many igneous rocks