Source and drain
Beginner
The two ends of a transistor switch. When the switch is on, electricity flows from one to the other.
Novice
Two heavily doped regions on either side of the gate. Carriers enter the channel at the source and leave at the drain. In an NMOS the source is the lower-voltage end; in a PMOS it is the higher-voltage end.
Expert
Physically symmetric in a standard MOSFET: which end is the source is set by the voltages, not the layout. Their junctions to the body add diffusion capacitance comparable to the gate capacitance.