Clipmap
A mipmap pyramid clipped to a small window around the viewer, so only the resolution needed near the camera stays in memory.
A full mipmap of a giant texture, or a whole landscape, will not fit in memory. A clipmap solves this by clipping each mip level to a small, movable square centred on the camera: high resolution up close, coarse in the distance, with the window sliding along as the viewer moves. Memory stays constant no matter how large the underlying data is.
Texture clipmaps were introduced by SGI (Tanner, Migdal and Jones, 1998). Hugues Hoppe and Frank Losasso carried the idea over to geometry as "geometry clipmaps" (2004) for terrain LOD. Related to virtual texturing, but driven by distance rather than visibility.