The Collaborative International Dictionary of English v.0.48:

Gypsum \Gyp"sum\ (j[i^]p"s[u^]m), n. [L. gypsum, Gr. gy`psos;
   cf. Ar. jibs plaster, mortar, Per. jabs[imac]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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gypsum
    n 1: a common white or colorless mineral (hydrated calcium
         sulphate) used to make cements and plasters (especially
         plaster of Paris)