Planar reflection
Exact reflections on a flat surface, made by mirroring the camera through the plane and rendering the scene a second time.
For water, mirrors and polished floors the method gives sharp, correct reflections: mirror the camera's position through the reflection plane, render the whole scene from that flipped view into a texture, then sample that texture on the surface. The cost is drawing the scene an extra time per reflective plane, so it's reserved for one or a few flat surfaces.
Curved or arbitrary surfaces are beyond it — there you fall back on reflection probes or screen-space reflections, both of which trade exactness for avoiding a whole extra render pass.