Nanite

The trick is to store source geometry as small clusters and, every frame, pick exactly the resolution each cluster needs — in practice roughly one triangle per pixel, no matter how dense the original model is. Tiny triangles are rasterized by a custom software rasterizer on the GPU, because fixed-function hardware falls apart once a triangle covers less than a pixel; larger surfaces take the usual path.

The payoff is that levels of detail and polygon budgets no longer need hand-tuning — you can import photogrammetry or ZBrush sculpts as-is. It shipped with UE5 Early Access in 2021. Early limits covered deforming and masked materials; support has widened since.