Definition
A secondary or supplementary system, component, or power source that supports the primary one. In aircraft, an auxiliary item is not the main means of performing a function, but it backs up, supplements, or extends the capability of the primary system.
Plain English
A backup or helper. It is not the main thing doing the job, but it adds to or supports the main thing when needed.
Context Anchor
Seen in terms such as auxiliary fuel pump, auxiliary fuel tank, auxiliary power unit, and auxiliary lighting.
Derivation
From the Latin auxilium, meaning 'help' or 'aid.' That origin matches the aviation use exactly: an auxiliary part is there to help the main system, not replace it.
Why Pilots Care
Knowing whether a system is primary or auxiliary tells you what you can rely on and what you can fall back to. If the main pump fails and an auxiliary pump is available, you have options. Confusing the two during a checklist or emergency can cost time you do not have.
Intuition Check
Auxiliary does not mean unimportant or merely optional. In aviation, an auxiliary system may be extra or backup, but it can still be required for a procedure or important for safety.
Example Sentence 1
He switched on the auxiliary fuel pump before starting the engine to make sure fuel pressure was steady.
Example Sentence 2
On the ramp the APU supplied auxiliary electrical power while the main engines were shut down.