a very soft, monoclinic mineral, CaSO·2HO, commonly found with other evaporites in sedimentary rock and used to make plaster of Paris and cement; hydrous calcium sulfate
See gypsum in American Heritage Dictionary 4
(jĭpˈsəm)
noun
A widespread colorless, white, or yellowish mineral, CaSO4·2H2O, used in the manufacture of plaster of Paris, various plaster products, and fertilizers.