Plain English Meaning

An APU is a small extra engine inside the aircraft whose only job is to make electricity and air pressure for the plane's systems, so the main engines don't have to be running to keep the lights, instruments, and air conditioning on.

Full Definition

Small onboard engines, typically gas turbines, fitted to many aircraft to supply electrical power and compressed air independently of the main engines. APUs allow systems such as avionics, lighting, air conditioning, and engine starting to operate while the aircraft is on the ground with the main engines shut down, and can also serve as a backup power source in flight on some aircraft.

Context Anchor

Seen in aircraft systems discussions, engine-start procedures, and operations at the or on the before the main engines are started.