Prism, in geometry, a solid bounded by plane faces, of which two that are opposite are equal, similar, and parallel, and are called the bases of the prism; the other surfaces are parallelograms. The axis is the line connecting the centres of the bases. The prism is triangular, square, pentagonal, and so on, according as the figure of the bases is triangular, square, pentagonal, etc. It is right or oblique according as the sides are perpendicular or oblique to the bases. A right prism is regular when its bases have the figure of a regular polygon. The prism corresponds among bodies with plane surfaces to the cylinder among bodies with curve surfaces. - In optics, a prism is a portion of a refracting medium bounded by two plane' surfaces inclined to one another. The line in which these two surfaces meet, or Would meet if produced, is the edge of the prism; their inclination is called its refracting angle. The form commonly used is a triangular prism of glass. A good contrivance for delicate experiments may be made with two rectangular pieces of plate glass firmly set to form two sides of a triangular box which is to be filled with water or spirits of turpentine.

The prism is essential in apparatus for decomposing light.