What does the abbreviation APU stand for?

The abbreviation APU stands for Auxiliary Power Unit. An APU is a crucial component in aircraft systems, serving as a small engine that provides power to start the main engines and to supply electrical power and pneumatic air for various onboard systems while the aircraft is on the ground. This allows for essential functions such as cabin air conditioning, lighting, and avionics without needing the main engines to be running, thus enhancing efficiency and safety during ground operations.

The other choices, although they contain plausible phrasing, do not accurately represent what APU stands for in aviation terminology. For example, an Air Purification Unit relates to filtering air but is not a standard term associated with aircraft operations. Automated Power Unit and Aeronautical Propulsion Unit also do not align with the established definitions used in the industry for APUs. Therefore, the term Auxiliary Power Unit correctly captures the function and significance of APUs in aviation.