The Collaborative International Dictionary of English v.0.48:

Prism \Prism\ (pr[i^]z'm), n. [L. prisma, Gr. pri`sma, fr.
   pri`zein, pri`ein, to saw: cf. F. prisme.]
   1. (Geom.) A solid whose bases or ends are any similar,
      equal, and parallel plane figures, and whose sides are
      parallelograms.
      [1913 Webster]

   Note: Prisms of different forms are often named from the
         figure of their bases; as, a triangular prism, a
         quadrangular prism, a rhombic prism, etc.
         [1913 Webster]

   2. (Opt.) A transparent body, with usually three rectangular
      plane faces or sides, and two equal and parallel
      triangular ends or bases; -- used in experiments on
      refraction, dispersion, etc.
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   3. (Crystallog.) A form the planes of which are parallel to
      the vertical axis. See Form, n., 13.
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   Achromatic prism (Opt.), a prism composed usually of two
      prisms of different transparent substances which have
      unequal dispersive powers, as two different kinds of
      glass, especially flint glass and crown glass, the
      difference of dispersive power being compensated by giving
      them different refracting angles, so that, when placed
      together so as to have opposite relative positions, a ray
      of light passed through them is refracted or bent into a
      new position, but is free from color.

   Nicol's prism, Nicol prism. [So called from Wm. Nicol, of
      Edinburgh, who first proposed it.] (Opt.) An instrument
      for experiments in polarization, consisting of a rhomb of
      Iceland spar, which has been bisected obliquely at a
      certain angle, and the two parts again joined with
      transparent cement, so that the ordinary image produced by
      double refraction is thrown out of the field by total
      reflection from the internal cemented surface, and the
      extraordinary, or polarized, image alone is transmitted.
      [1913 Webster] Prismatic