Definition
A portable or fixed electrical power source, separate from the aircraft, that supplies direct current to the airplane through an external receptacle so electrical systems and engine starting can be performed without drawing on the aircraft battery.
Plain English
A power supply on the ground that you plug into the airplane to run its electrical systems or help start the engine, so you don't drain the aircraft's own battery.
Context Anchor
You may see a GPU during preflight, maintenance, engine start, or other ground operations when the aircraft is plugged into outside electrical power.
Derivation
External means coming from outside the aircraft. Ground refers to its use while the airplane is on the ground. Power unit simply means a device that supplies electrical power. Together the term describes exactly what it is: an outside, on-the-ground source of power.
Why Pilots Care
Allows preflight checks and maintenance while saving battery power and reducing engine wear.
Analogy
It is similar to plugging a parked vehicle into an outside power source so you can run electrical equipment without draining the vehicle’s own battery.
Intuition Check
Do not think of the GPU as part of the aircraft. It is separate ground equipment that temporarily feeds power into the aircraft while the aircraft is parked.
Example Sentence 1
On a cold morning, the line crew connected an external ground power unit to the airplane so the pilot could start the engine without straining the battery.
Example Sentence 2
Maintenance used a GPU to keep the electrical systems running during a long inspection without draining the aircraft battery.