From LightWiki

Technically speaking a frustum is the meat between two slices of a solid - imagine a ball with two parallel sheets of glass pushed through it. The bit left between the two sheets is the frustum. Normally the term is applied to the truncated cone emitted from a light, or the in-focus view from a camera (the portions that are out of focus would be on either side of the two planes cutting through the camera's field of view). Frustrum is considered a variant spelling.

Image showing camera frustum. This is only applicable to simple projections. For example, an Orthographic camera would have a cubic 'frustum' and a fisheye camera would have curved sides.