Source : Webster's Revised Unabridged Dictionary (1913)

Gypsum \Gyp"sum\, n. [L. gypsum, Gr. ?; cf. Ar. jibs plaster,
   mortar, Per. jabs?n lime.] (Min.)
   A mineral consisting of the hydrous sulphate of lime
   (calcium). When calcined, it forms plaster of Paris. Selenite
   is a transparent, crystalline variety; alabaster, a fine,
   white, massive variety.

Source : WordNet®

gypsum
     n : a common white or colorless mineral (hydrated calcium
         sulphate) used to make cements and plasters (especially
         plaster of Paris)