Also called "stove plug." A 30 or 50A, 125/250V 3-pole 4-wire grounding, parallel-blade wiring device sometimes used to power small dimmer racks or distros. The most common are the NEMA 14-30 (pictured below) and the NEMA 14-50, although there are many other variants.
Leviton 30Amp 125V/50Amp 250V Dual Power Attachment Plugs NEMA 14-30P/14-50P - Commercial Plugs - Leviton 275-T - LevitonProducts.com
Leviton 30Amp 125V/50Amp 250V Dual Power Attachment Plugs NEMA 14-30P/14-50P - Commercial Plugs - Leviton 275-T - LevitonProducts.com