Screen-space error
How many pixels a simplified model would deviate from the original on screen — the metric most LOD systems pick levels by.
When a mesh is simplified, its geometric error is measured in world units: the furthest the simplified surface strays from the real one. That number says nothing about whether anyone will notice. Project it through the camera — divide by distance, multiply by screen height, divide by the tangent of half the field of view — and you get the error in pixels. If it falls under a threshold, say one pixel, the coarser level can be used and nobody is the wiser.
The advantage over switching at fixed distances is that one rule works regardless of resolution, field of view or object size. Terrain algorithms such as ROAM and chunked LOD were built on the idea, Cesium's 3D Tiles stores a geometricError per tile, and Nanite in Unreal Engine 5 selects triangle clusters so the error stays around a single pixel.