Source : Webster's Revised Unabridged Dictionary (1913)

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
   lemonyellow 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.

         Our orpiment and sublimed mercurie.      --Chaucer.

   {Red orpiment}, realgar; the red sulphide of arsenic.

   {Yellow orpiment}, king's yellow.

Source : WordNet®

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