Body (substrate or well)
Beginner
The silicon a transistor is built in. It is usually just held at one fixed voltage.
Novice
The fourth terminal: the doped silicon under the channel. NMOS bodies are usually tied to ground and PMOS bodies (an n-type well) to the supply voltage.
Expert
Its voltage relative to the source shifts the threshold (the body effect). Body ties come from well taps in the layout; deliberate body biasing tunes threshold and leakage in some processes.