1. (in mathematics) A polyhedron with two parallel congruent polygons as bases and parallelograms for all other faces. A triangular prism has triangular bases.
2. (in optics) A block of glass or other transparent material, usually having triangular bases. Prisms have several uses in optical systems: they can be used to deviate a ray, to disperse white light into the visible spectrum, or to erect an inverted image (see binoculars). Prisms of other materials are used for different kinds of radiation. See also Nicol prism; Wollaston prism.