What is Auxiliary?

Auxiliary definition and meaning on Dictionary terms:
additional; supplementary; reserve: an auxiliary police force.
used as a substitute or reserve in case of need: The hospital has an auxiliary power system in case of a blackout.

(of a boat) having an engine that can be used to supplement the sails: an auxiliary yawl.
giving support; serving as an aid; helpful: The mind and emotions are auxiliary to each other. Passion is auxiliary to art.

noun, plural aux