A prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use "prism" usually refers to this type.
Prisms are typically made out of glass, but can be made from any material that is transparent to the wavelengths for which they are designed.
A prism can be used to break light up into its constituent spectral colors (the colors of the rainbow). Prisms can also be used to reflect light, or to split light into components with different polarizations.