Plaster, a pasty composition that hardens on drying and is used for coating interior walls, ceilings, and partitions. Plaster is generally composed of sand, water, and a cementing agent such as gypsum, lime, or portland cement. When using plaster to coat a surface, hair or fiber is mixed with the first and second coats to strengthen the plaster. Wallboard, which is used as a substitute for plastering, is a large sheet of prefabricated material made of fiberboard, paper, or felt with a hardened gypsum plaster core. The term plaster is sometimes also applied to molded ornamental walls and ceilings (see Stucco) and to plaster of Paris.