View frustum
The truncated pyramid of space a perspective camera can see — bounded by six planes, and anything outside them can be thrown away before it is drawn.
A perspective camera doesn't see a box but a truncated pyramid: the apex at the eye, the base at the far clip plane, sliced off near the camera by the near plane. Its shape is set by the field of view, the aspect ratio and the distance to the two clip planes. The projection matrix maps this pyramid into the cube that becomes clip space.
Its six planes — top, bottom, left, right, near and far — are the basis of frustum culling: anything whose bounding volume lies entirely outside one plane can be skipped without further testing, the cheapest big win a renderer gets. Level-of-detail selection often keys off the same distances.