Render target
An off-screen buffer the GPU draws into instead of the screen, so the result can be read back as a texture.
Rasterization normally lands in the screen's framebuffer. A render target is instead a texture you bind as the destination of a pass and then sample in the next one — the basis for everything not drawn straight to the display: shadow maps, mirrors, post-processing, and the G-buffer that deferred shading fills. In OpenGL the mechanism is the framebuffer object (FBO); Unity calls it a render texture.
MRT (multiple render targets) lets a single pass write to several render targets at once — exactly what makes deferred shading possible without redrawing the geometry over and over. The cost is bandwidth and VRAM: every full-resolution target adds up, and mobile GPUs with tile-based architectures punish needless reads back from memory hard.