LINK / CITE ADD TO WORD LIST. noun. The definition of a prism is a solid geometric object (especially one made of glass) with sides that are parallelograms and end faces that are parallel and equal in shape and size, or anything that refracts light. An example of a prism is a glass object in a specific geometric shape that makes white light to show all the colors of the rainbow. Geom. a solid figure whose ends are parallel, polygonal, and equal in size and shape, and whose sides are parallelograms.