An auxiliary is a person who is employed to assist other people in their work. Auxiliaries are often medical workers or members of the armed forces.

Auxiliary staff and troops assist other staff and troops.

Auxiliary equipment is extra equipment that is available for use when necessary.

An auxiliary is an organization that is connected with, but less important than, another organization; for example, an organization for the wives of the members of the main organization.

In grammar, an auxiliary or auxiliary verb is a verb which is used with a main verb, for example to form different tenses or to make the verb passive. In English, the basic auxiliary verbs are 'be', 'have', and 'do'. Modal verbs such as 'can' and 'will' are also sometimes called auxiliaries.