Definition 1 of 2
Definition
A pilot-operated switch in the cockpit that connects or disconnects the aircraft battery (and usually the alternator or generator) to the airplane's electrical system. When ON, electrical power is available to operate items such as radios, lights, fuel gauges, flaps, and the starter. When OFF, the electrical system is isolated from the battery. On many light aircraft it is a split rocker switch with separate BAT and ALT halves that can be operated together or independently.
Plain English
The main switch that turns the airplane's electrical system on and off. With it on, the battery powers things like the radios, lights, and gauges. With it off, the electrical system is dead.
Context Anchor
You encounter the master switch during preflight checks, before engine start, and during shutdown when confirming the airplane’s electrical power is on or off as required.
Derivation
"Master" here means "main" or "controlling" — the one switch that controls all the others downstream. Same sense as a "master key" that opens everything.
Why Pilots Care
It allows the pilot to isolate electrical problems, secure the aircraft after flight, or prevent battery drain when the engine is not running.
Intuition Check
Do not assume the master switch starts or stops the engine. It controls the airplane’s main electrical power; engine start and engine ignition are separate functions.
Example Sentence 1
Before checking the fuel gauges and lowering the flaps during preflight, she turned the master switch on.
Example Sentence 2
Upon detecting an electrical odor in flight, the pilot immediately turned off the master switch to remove all electrical power.