Plain English Meaning
The knob or lever in the cockpit that decides which fuel tank the engine is drawing fuel from, or shuts the fuel off entirely.
Full Definition
A cockpit-mounted valve control used to choose which fuel tank feeds the engine. Typical positions include LEFT, RIGHT, BOTH, and OFF, depending on the airplane's fuel system design.
Context Anchor
You check the tank selector before starting the engine, including before hand propping, so the engine has fuel from the intended tank.
Why Pilots Care
Correct tank selection maintains continuous fuel flow, prevents engine stoppage from an empty tank, and avoids fuel imbalance or contamination.