A term from traditional photography, it is the shape that the viewer's FOV creates when projected into the scene. This pyramidal shape defines the limits of what the viewer can see, so it is used to calculate clipping, collisions, fog, etc. Imagine your eyeball stuck to the top of a clear pyramid, looking straight down into it. The top-most point that is scratching against your eyeball is the near end of the frustum. You cannot see the side faces of the pyramid, because they are perpendicular to your eyeball, but you can see everything within the pyramid. In photography, this pyramid is infinitely tall, because in theory you can see to infinity. In RT3D, the frustum is not infinitely long. It has near and far clipping planes to help reduce polygon counts. The near clipping plane cuts off the very top of the pyramid, while the far clipping plane defines the base of the pyramid.

This definition is in context to Real Time 3D. See more contextual defintions for Frustum.