Definition
A position on the fuel selector valve that shuts off fuel flow from all tanks to the engine. Selecting OFF isolates the fuel system from the engine and is used during certain emergencies (such as an engine fire) or when securing the aircraft.
Plain English
The setting on the fuel selector that stops fuel from reaching the engine. With it in this position, the engine cannot run because no fuel can get through.
Context Anchor
Seen on fuel selectors, fuel shutoff valves, and checklist items for engine start, shutdown, emergency procedures, and securing the airplane.
Derivation
Off comes from Old English words meaning away or away from. That helps here because OFF means the fuel is taken away from the engine by closing the path it would normally flow through.
Why Pilots Care
Used to isolate fuel during emergencies such as engine fire or when securing the aircraft on the ground.
Intuition Check
Do not read OFF as just a general label meaning something is not being used. In this fuel-system context, OFF means the fuel path is closed and fuel flow to the engine is stopped.
Example Sentence 1
During the engine fire checklist, the pilot moved the fuel selector to OFF to stop fuel flow to the engine.
Example Sentence 2
During the engine fire checklist the first step was to turn the fuel selector OFF.