the body diode

Hidden inside every power MOSFET is a diode you did not ask for. Because the body (substrate) is tied to the source, the junction between that body and the drain forms a built-in PN diode, called the body diode or intrinsic diode. For an N-channel part it points from source to drain, so it conducts whenever the drain is pulled more negative than the source, even when the gate is off and the channel is closed. It is an unavoidable side effect of how the device is built.

Far from being only a nuisance, the body diode is genuinely useful in switching circuits that drive inductive loads. When you switch off a motor or coil, its current cannot stop instantly, and the body diode of the right MOSFET provides a path for that current to freewheel safely, much like an external flyback diode. In a half-bridge or motor driver, the two MOSFETs' body diodes catch the inductor current during the brief dead time when both transistors are off.

The honest caveat is that a MOSFET body diode is a mediocre diode: it has a relatively high forward drop and, worse, it is slow to stop conducting (it has significant reverse-recovery charge). In fast or high-frequency switching that slowness causes extra loss and current spikes. So in demanding converters designers often add a faster external Schottky diode in parallel, or pick a MOSFET with a fast, soft body diode. Never assume the body diode is as good as a purpose-built one.