Definition of "auxiliary"

Auxiliaries and ancillaries are comprehensive, and include a Westinghouse motor-driven recriprocating compressor used for locomotive braking and general service air, two rotary exhauster sets for train brakes when hauling passenger or fitted freight trains, and an oil-fired train heating boiler.

1950 November, “Mixed-Traffic Diesel-Electric Locomotives for Ireland”, in Railway Magazine, page 781

A drive to exploit the Highlands as a winter sports area, with all the necessary auxiliaries, such as chair lifts in the Cairngorms, shows increasing promise, [...].

1962 September, G. Freeman Allen, “The New Look in Scotland's Northern Division—1”, in Modern Railways, page 163

The three traditionally recognized Non-modal Auxiliaries are the per-fective Auxiliary have, the progressive Auxiliary be, and the passive Auxiliarybe. Perfective have is so-called because it marks the completion (hence, perfec-tion) of an action; it is followed by a VP headed by a perfective -n participle, asin:(121) The referee has [VP shown him the red card]

1988, Andrew Radford, Transformational grammar: a first course, Cambridge, UK: Cambridge University Press, page 151