Progressive mesh
A base mesh plus an ordered list of vertex splits, replayed forward or backward to give continuous level of detail.
Hugues Hoppe introduced the format at SIGGRAPH 1996. The idea: simplify a mesh all the way down to a coarse base with repeated edge collapses, storing each collapse as a reversible vertex split. Applying more or fewer splits yields any polygon count between the base and the original — a continuous LOD chain instead of a handful of discrete steps that pop visibly.
The format also suits progressive transmission: send the base first, the refinements after. The drawback is that each frame's adjustment costs CPU work, which is why modern engines more often bake discrete LOD levels ahead of time.