Plain English Meaning
A switch or knob in the cockpit that lets the pilot choose which fuel tank the engine draws from, or shut the fuel off altogether.
Full Definition
A cockpit-operated valve that controls which fuel tank (or tanks) supplies fuel to the engine. The pilot sets it to a specific tank position (e.g., LEFT, RIGHT, BOTH) or to OFF to shut off fuel flow entirely.
Context Anchor
Seen during preflight checks, normal tank management in flight, and engine-failure or emergency checklist actions.