Wiktionary
three-way switch
n. A type of light switch which can be arranged in pairs to control lights from two locations, as at either end of a stairway or long hallway.
WordNet
three-way switch
n. an electric switch that has three terminals; used to control a circuit from two different locations [syn: three-point switch]
Usage examples of "three-way switch".
The lamp featured a three-way switch, and Bobby clicked it to the lowest level of light.
One lamp had been set at the lowest intensity on its three-way switch.