HLOD
Hierarchical LOD — distant objects are merged into a single simplified proxy mesh to slash draw-call counts at long range.
Ordinary LOD swaps a mesh for a cheaper version with distance, but each object is still drawn on its own. HLOD goes further and bakes whole clusters of distant objects — a city block, a forest — into one shared proxy mesh with a merged atlas texture. Hundreds of draw calls become one, and the per-object overhead vanishes at the range where the detail was invisible anyway.
The price is memory and bake time: the proxy is precomputed offline and takes space. Unreal Engine ships a built-in HLOD system, and the technique belongs to open worlds where sightlines are long. UE5's Nanite, which streams geometry per triangle, makes manual HLOD less necessary for mesh detail specifically.