Render to texture
Rendering a scene or sub-pass into an off-screen texture so the result can be fed back as input to a later pass.
Instead of drawing straight to the screen buffer, you bind a texture as the target — in OpenGL through a framebuffer object (FBO), standardised in 2004. Once the pass finishes, the texture can be sampled like any other image. It is the building block of all multi-pass rendering.
The uses are everywhere: mirrors and security-camera monitors, dynamic shadow maps, reflection probes, minimaps, the whole G-buffer of deferred rendering, and every post-processing effect that reads the previous frame. Before FBOs existed you copied the frame into a texture with glCopyTexImage — slower and clumsier.