A prism is a three-dimensional solid object that is bound on all its sides by plane faces. It is a polyhedron comprising an n-sided polygon base, a second base which is a translated copy of the first, and n other faces, necessarily all parallelograms, joining corresponding sides of the two bases). The top and bottom faces of a prism are identical and are called bases, while the faces other than the top and bottom of a prism are called its lateral faces. Prisms are named after their bases, for example, a prism with a pentagonal base is called a pentagonal prism).

Prisms are used in optics to analyze and reflect light. An ordinary triangular prism can separate white light into its constituent colors, called a spectrum. Each color, or wavelength, making up the white light is bent, or refracted, a different amount; the shorter wavelengths (those toward the violet end of the spectrum) are bent the most, and the longer wavelengths (those toward the red end of the spectrum) are bent the least. Prisms of this kind are used in certain spectroscopes, instruments for analyzing light and for determining the identity and structure of materials that emit or absorb light. Prisms can also reverse the direction of light by internal reflection, and for this purpose, they are useful in binoculars.

In summary, a prism is a solid shape that has two parallel bases and is bound on all its sides by plane faces. It is used in optics to analyze and reflect light.