The Collaborative International Dictionary of English v.0.48:

Plaster \Plas"ter\, n. [AS., a plaster (in sense 1), fr. L.
   emplastrum, Gr. ?, ?, fr. ? to daub on, stuff in; ? in + ? to
   mold: cf. OF. plastre a plaster (in sense 2), F. pl[^a]tre.
   Cf. Plastic, Emplaster, Piaster.] [Formerly written
   also plaister.]
   1. (Med.) An external application of a consistency harder
      than ointment, prepared for use by spreading it on linen,
      leather, silk, or other material. It is adhesive at the
      ordinary temperature of the body, and is used, according
      to its composition, to produce a medicinal effect, to bind
      parts together, etc.; as, a porous plaster; sticking
      plaster.
      [1913 Webster]

   2. A composition of lime, water, and sand, with or without
      hair as a bond, for coating walls, ceilings, and
      partitions of houses. See Mortar.
      [1913 Webster]

   3. Calcined gypsum, or plaster of Paris, especially when
      ground, as used for making ornaments, figures, moldings,
      etc.; or calcined gypsum used as a fertilizer.
      [1913 Webster]

   Plaster cast, a copy of an object obtained by pouring
      plaster of Paris mixed with water into a mold.

   Plaster of Paris. [So called because originally brought
      from a suburb of Paris.] (Chem.) Anhydrous calcium
      sulphate, or calcined gypsum, which forms with water a
      paste which soon sets or hardens, and is used for casts,
      moldings, etc. The term is loosely applied to any plaster
      stone or species of gypsum.

   Plaster of Paris bandage (Surg.), a bandage saturated with
      a paste of plaster of Paris, which on drying forms a
      perfectly fitting splint.

   Plaster stone, any species of gypsum. See Gypsum.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plaster of Paris
    n 1: any of several gypsum cements; a white powder (a form of
         calcium sulphate) that forms a paste when mixed with water
         and hardens into a solid; used in making molds and
         sculptures and casts for broken limbs [syn: plaster of
         Paris, plaster]