Frustum

A frustum is the portion of a solid (normally a cone or pyramid) which lies between two parallel planes cutting the solid. Degenerate cases are obtained for finite solids by cutting with a single plane only.

An example of a pyramidal frustum may be seen on the reverse of the Great Seal of the United States, as on the back of the U.S. one-dollar bill. The “unfinished pyramid” is surmounted by the “eye of providence.”

Certain ancient Native American mounds also take on the frustum of a pyramid.

The focal field of a still or video camera forms a frustum. In three-dimensional rendering, frustum culling involves ignoring those portions of the image(s) outside the frustum in order to process the relevant visual data more efficiently.

The spelling frustrum, although strictly an error, is becoming frequent enough to be considered a variant.