prism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A solid figure whose bases or ends have the same size and shape and are parallel to one another, and each of whose sides is a parallelogram.
• noun A transparent body of this form, often of glass and usually with triangular ends, used for separating white light passed through it into a spectrum or for reflecting beams of light.
• noun A cut-glass object, such as a pendant of a chandelier.
• noun A crystal form consisting of three or more similar faces parallel to a single axis.
• noun A medium that misrepresents whatever is seen through it.

from The Century Dictionary.

• noun In geometry, a solid whose bases or ends are any similar, equal, and parallel plane polygons, and whose sides are parallelograms.
• noun Specifically An optical instrument consisting of a transparent, medium so arranged that the surfaces which receive and transmit light form an angle with each other: usually of a triangular form with well-polished sides, which meet in three parallel lines, and made of glass, rock-salt, or quartz, or a liquid, as carbon disulphid, contained in a prismatic receptacle formed of plates of glass.
• noun In crystallography, a form consisting of planes, usually four, six, eight, or twelve, which are parallel to the vertical axis.
• noun In canals, a part of the water-space in a straight section of a canal, considered as a parallelepiped.
• noun In weaving, same as pattern-box
• noun A form of illuminator consisting of a prism with two convex surfaces, by which the light is brought to a focus upon the object.
• noun According to some authors any form having two pairs of parallel faces is called a prism; in this sense the term includes the domes of the orthorhombic system (this name being then restricted to a form having two faces only intersecting in an edge) and the hemipyramids of the monoclinic system.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) A solid whose bases or ends are any similar, equal, and parallel plane figures, and whose sides are parallelograms.
• noun (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.
• noun (Crystallog.) A form the planes of which are parallel to the vertical axis. See Form, n., 13.
• noun (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.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geometry A polyhedron with parallel ends of the same size and shape, the other faces being parallelogram-shaped sides.
• noun A transparent block in the shape of a prism (typically with triangular ends), used to split or reflect light.
• noun A crystal in which the faces are parallel to the vertical axis.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a polyhedron with two congruent and parallel faces (the bases) and whose lateral faces are parallelograms
• noun optical device having a triangular shape and made of glass or quartz; used to deviate a beam or invert an image