Screen space reflections (SSR)
Faking reflections by marching rays through the depth and colour buffers and reusing already-rendered pixels, instead of re-rendering the scene from the mirror's view.
For each pixel a reflection ray is cast and stepped forward in screen space. Where it hits a previously drawn surface — according to the depth buffer — that colour is read off and used as the reflection. It all reuses information already sitting in the G-buffer, so the cost is low and the reflections are fully dynamic with no precomputation.
The limitation follows straight from the name: only what is visible on screen can be mirrored. Objects off-screen, behind the camera or at grazing angles have no data, which gives the characteristic fade toward the edges and reflections that suddenly vanish. Popularised around 2011 (Crytek, Killzone Shadow Fall) and nearly always paired with a fallback for what SSR misses. A screen-space special case of global illumination.