Plain English Meaning
A plug-in point on the outside of the aircraft where you can connect an external power source to start the engine or run electrical systems on the ground.
Full Definition
A socket installed on the airframe that allows a ground power unit (GPU) or auxiliary battery cart to be plugged into the aircraft's electrical system, supplying power for engine start or ground operations without drawing on the aircraft battery.
Context Anchor
Seen during engine starting, maintenance, and ground operations when outside electrical power is connected to the aircraft.