Noun
auxiliary power unit (plural auxiliary power units)
(aviation) A generator in aircraft or other vehicles for providing electric power to various systems, such as lighting, especially when the main engines are not running.
A simple auxiliary power unit can allow a trolleybus to get around a route blockage or can reduce the amount (or complexity) of overhead wiring needed at operating garages (depots). Source: Internet
Other modifications to the aircraft also included installing an auxiliary power unit, crew luggage compartment, sensor operator console, flight following console and upgraded avionics. Source: Internet
The auxiliary power unit enables the aircraft to start engines and provides electrical power and cabin heat independent of ground support equipment. Source: Internet
The vehicle also contains a 2-stroke diesel powered auxiliary power unit that can power all non-automotive energy requirements on the Field Artillery Ammunition Supply Vehicle and on the howitzer when a slave cable is used to connect the two. Source: Internet