What Is APU? Definition & Meaning in Aviation
An APU (auxiliary power unit) is a small engine, usually in the tail, that supplies electricity and air conditioning when the main engines are off.
In plain language
It lets an aircraft stay cool and powered at the gate and start its main engines. Airports often supply ground power so that the APU can be shut down.
Example
In hot weather at a gate, the APU or ground equipment keeps the cabin cool while passengers board.
Why it matters
The APU explains the hum you hear at the gate and why airports try to provide ground power. It also keeps the cabin comfortable when the main engines are off.