Auxiliary verb

An auxiliary (verb) is a verb-like function word that combines with a main verb and typically helps to express various concepts of tense, aspect or mood, but may also be used for other functions such as valence-changing operations, negation, or various notions that are expressed by adverbs in other languages.


 * "For a number of authors, including the present one, the use of the term auxiliary is primarily associated with a limited range of notional domains, most of all with the domains of tense, aspect, and modality... In quite a few works, however, the term is applied to a much wider range of grammatical and/or lexical phenomena including expressions serving derivative, pro-verb-like, adpositional and other purposes...and, in fact, up to now there does not appear to be any typologically meaningful way of delimiting the range of conceptual, morphosyntactic or other properties that an auxiliary, or a "canonical auxiliary" for that matter, should exhibit..." (Heine 1993:5)
 * "'Auxiliary verb' is here considered to be an item on the lexical verb–functional affix continuum, which tends to be at least somewhat semantically bleached, and grammaticalized to express one or more of a range of salient verbal categories, most typically aspectual and modal categories, but also not infrequently temporal, negative polarity, or voice categories." (Anderson 2006:4-5)

Term properties
Auxiliary verb and its short form auxiliary are used interchangeably, because grammarians do not talk about "auxiliary nouns", "auxiliary adjectives", or other kinds of auxiliary words.

Examples
is going, has gone, will go, may go, doesn't go 

Synonym

 * helping verb

Origin
The adjective auxiliary means 'helping' (Latin auxilium 'help'): Auxiliaries "help" the main verb express certain notions that cannot be expressed directly with inflectional means on it. The term has been attested in English since the 18th century.

Link
Utrecht Lexicon of Linguistics

Other languages
German Hilfsverb, Auxiliarverb