Webster's English Dictionary

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gypsum

gyp.sum \'jip-s*m\ n [L, fr. Gk gypsos, of Sem origin; akin to Ar jibs 
   plaster] 1: a widely distributed mineral CaSo4.2H2O consisting of hydrou s 
   calcium sulfate that is used esp. as a soil amendment and in making plaster 
   of paris 2: PLASTERBOARD