Imposters are an extreme form of LOD (level of detail) optimization, replacing entire models with simple, flat pictures of themselves. They are much like the flat facades used in old Hollywood films, or the cardboard character cutouts used to promote books and movies. Here is an Overgrowth scene that has 3D trees interspersed with imposter trees: first you can see flat checkerboard imposters, then cutout checkerboard imposters, and finally the actual colored and shaded imposters.

Source: Imposters – Wolfire Games Blog