LOD - Level of Detail. This refers to the reduction or the increase of the complexity of a mesh, usually at different viewing distances.
See also Mipmapping.
The next picture is a grid for demonstrating a quadtree-based LOD system.
He moved to a point on the ground, froze the LOD state, then backed off into space where you can see the effect. The LOD parameters were also modified to make the effect more visible.