Impostor
A billboard that replaces distant 3D geometry with a pre-rendered image pasted onto a single quad that turns to face the camera.
It is LOD at its most aggressive. A whole tree or building boils down to two triangles once it is far enough away that nobody can tell the difference anyway. Distant forests and racing tracks are often made almost entirely of impostors — SpeedTree built a whole industry on the trick.
The difference from a plain billboard is that the texture is generated from the model itself, not a hand-drawn flat sprite. When the viewer's angle drifts too far from the one the image was rendered at, the fake becomes visible, so the impostor is re-rendered against a new view — either ahead of time in a grid of angles, or on demand. The trade-off is the classic one: more memory for the textures against fewer triangles to draw.