Plain English Meaning
A switch or knob in the cockpit that lets the pilot pick which fuel tank the engine drinks from.
Full Definition
A cockpit-mounted valve control that allows the pilot to choose which fuel tank, or combination of tanks, supplies fuel to the engine. Common positions include LEFT, RIGHT, BOTH, and OFF, though exact positions vary by aircraft.
Context Anchor
You will see or use the fuel tank selector during preflight checks, before takeoff, in cruise fuel management, and during engine-failure or emergency landing procedures.