Temporal reprojection
Reuses the previous frame's result by reprojecting it into the current one, spreading expensive effects across several frames.
Using motion vectors and depth, you work out where each pixel sat in the previous frame and fetch its value from there. The cost of anti-aliasing, ambient occlusion or reflections is amortised over time rather than paid in full every frame — the basis of TAA and of temporal upscalers like DLSS and FSR 2.
The catch is history that is missing or wrong: newly revealed pixels have no past (disocclusion), and when something moves slightly off, a smeared ghost trails behind it. The usual fixes are to clamp the fetched colour against its neighbourhood in the new frame and to reject history that looks suspect.