A large power tap that lighting, sound, automation, and motors can be "tied-in" to. Usually in the theatre they come in 100, 200, and 400 amp varieties, and are often located on the upstage side of the proscenium wall and/or offstage/upstage. They usually are 120/208VAC, 3Ø WYE-connected, 5 wire (red, blue, black, green "ground", and white "neutral") 3 phase. They usually have a large circuit breaker, or 3 cartridge fuses, attached to E1016 Camlok-style receptacles, or more commonly lugs that "tails" (bare stripped cable at one end and female connector at the other) can be connected to. Five feeder cables, usually 4/0 (pronounced four-aught), are used to carry the power to a power distribution panel ("distro") or a dimmer rack. Standard sizes are 100A, 200A, and 400A (per leg). See also disconnect.
Modern types have safety features built in that will cut the power from flowing if any of the doors are opened or leads removed, to avoid injury and/ or death from dangerous high voltage.
One purpose-built example from Lex Products-PowerGATE.
Another from http://www.unionconnector.com/cswitch/index.htm.
And since it was the first hit returned by Google:
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See also the thread http://www.controlbooth.com/forums/lighting-electrics/25748-safe-alternatives-company-switch.html , particularly