Geomorphing
A smooth transition between two terrain LOD levels by gradually interpolating vertex heights, so the detail swap does not visibly "pop".
When a terrain patch changes level of detail, vertices appear or vanish and the seam shows up as a snap — a sudden shift in the silhouette in the middle of the frame. Geomorphing hides the snap by letting a vertex that is about to be removed slide toward its position in the coarser level over a few frames or metres, turning a discrete swap into a continuous one.
The technique came out of the terrain literature around the turn of the century — Hugues Hoppe's progressive meshes (1996) and Willem de Boer's geomipmapping (2000) — and suits heightmap-based, chunked terrain. The interpolation is cheap enough to run in the vertex shader every frame.