Definition
An electrical switch with three terminals that allows a single light or load to be controlled from two separate locations. Each switch has one common terminal and two traveler terminals, and the position of either switch can complete or break the circuit.
Plain English
A switch wired so that two different switches can each turn the same light on or off, no matter which one was used last.
Context Anchor
Seen in aircraft electrical systems, wiring diagrams, and lighting troubleshooting.
Derivation
Called 'three-way' because the switch has three connection points (terminals), not because it has three positions. The name describes the wiring, not the switch action.
Why Pilots Care
Correct understanding prevents misdiagnosis of lighting faults during preflight or maintenance and ensures reliable cabin illumination for night operations.
Analogy
It is like a hallway light at home that can be turned on at one end of the hall and turned off at the other end.
Intuition Check
Do not read “three-way” as three brightness levels or three switch positions. Here it means one light can be controlled from two different places.
Example Sentence 1
The cabin dome light is wired through three-way light switches so passengers can turn it off from either the front or rear door.
Example Sentence 2
During the night preflight the pilot confirmed both three-way switches for the instrument lights were functioning from each seat.