a prism with two congruent triangles and three congruent rectangles

In geometry, a triangular prism or three-sided prism is a polyhedron made of a triangular base, a translated copy, and 3 faces joining corresponding sides.

In optics, a triangular prism is a type of optical prism with the shape of a geometrical triangular prism. It is the most widely-known type of optical prism, although perhaps not the most common in actual use. Triangular prisms are used to disperse light, that is to break light up into its spectral components.