A geometrical figure with volume counting with a pair of parallel bases.
The most evocative prism has triangular bases (e.g. Post #742744). It is popular because of its ability to create [[rainbow|rainbows]] out of ordinary (polichromatic) light.

h5. Clarification

Cubes [b]are[/b] prisms too, but those  are better off tagged as [[cube|cubes]].
Pyramids [b]are not[/b] prisms.
prism