The Collaborative International Dictionary of English v.0.48:

Orpiment \Or"pi*ment\, n. [F., fr. L. auripigmentum; aurum gold
   + pigmentum pigment. Cf. Aureate, Pigment, Orpin,
   Orpine.] (Chem.)
   Arsenic sesquisulphide, produced artificially as an amorphous
   lemon-yellow powder, and occurring naturally as a yellow
   crystalline mineral; -- formerly called auripigment. It is
   used in king's yellow, in white Indian fire, and in certain
   technical processes, as indigo printing.
   [1913 Webster]

         Our orpiment and sublimed mercurie.      --Chaucer.
   [1913 Webster]

   Red orpiment, realgar; the red sulphide of arsenic.

   Yellow orpiment, king's yellow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

orpiment
    n 1: a yellow mineral occurring in conjunction with realgar; an
         ore of arsenic