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aux·​il·​ia·​ry ȯg-ˈzil-yə-rē How to pronounce auxiliary (audio)
-ˈzi-lə- How to pronounce auxiliary (audio)
: offering or providing help
: functioning in a subsidiary capacity
an auxiliary branch of the state university
of a verb : accompanying another verb and typically expressing person, number, mood, or tense
In "I will go," the word "will" is an auxiliary verb.
: supplementary
a telescope with its auxiliary apparatus
: constituting a reserve
an auxiliary power plant
an auxiliary police force
of a boat : equipped with sails and a supplementary inboard engine
an auxiliary sloop


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plural auxiliaries
: an auxiliary person, group, or device
specifically : a member of a foreign force serving a nation at war
or auxiliary bishop : a Roman Catholic titular bishop assisting a diocesan bishop and not having the right of succession
: a sailing boat or ship equipped with a supplementary inboard engine
or auxiliary verb grammar : a verb (such as have, be, may, do, shall, will, can, or must) that is used with another verb in a verb phrase to show tense, to form a question, etc. : helping verb
In "I will go," the verb "will" is an auxiliary.

Note: The verbs have and be are used as auxiliaries to form the perfect tenses ("has worked," "had worked," "have been working," etc.) and the progressive tenses ("is talking," "was talking," "have been talking," etc.), respectively. The verb do functions as an auxiliary mainly in negative sentences ("I don't want to go"), in questions ("do you want to go?"), and in emphatic statements ("I really do want to go"). Do can also substitute for another verb that has just been stated ("I went, and so did she"). Another kind of auxiliary called a modal verb is used to change a verb's meaning to something other than simple fact (as to indicate permission, ability, prediction, possibility, or necessity). In the examples "it must be almost 10:00," "we can arrive early," and "may I call you tomorrow?" the verb forms must, can, and may are functioning as modal auxiliaries.

Did you know?

What would we do if you sang out of tune—would we stand up and walk out on you? Not likely! Instead, we would provide auxiliary harmonies, joining our voices with yours in support. And if you need a little help from your friends in understanding the meaning of auxiliary, we’re here for that, too—just lend us your ears. Auxiliary, which comes from the Latin noun auxilium, meaning “aid,” “assistance,” or “reinforcement,” is used in a wide range of capacities in English to describe a person or thing that assists another. A fire department may bring in auxiliary units, for example, to battle a tough blaze, or a sailboat may be equipped with an auxiliary engine to supply propulsion when the wind disappears. In grammar, an auxiliary verb assists another (main) verb to express person, number, mood, or tense, such as have in “They have now been informed about the meaning of auxiliary.” Isn’t auxiliary fab?

Did you know?

What is an auxiliary verb?

An auxiliary verb, also called a helping verb, is a verb that is used with another verb (or two other verbs) in a verb phrase.

The auxiliary verbs (which include be, have, do, and the modal verbs like may, shall, will, can, and must) combine with verbs to do things like show a verb's tense or form a question.

Be, have, and do are used as both independent verbs and as auxiliary verbs.

Have is used to form all the perfect tenses, the tenses that are used to refer to an action or state that is completed at the time of speaking or at a time spoken of:

I have packed the car. (present perfect)

I had packed my suitcase the night before. (past perfect)

I will have packed everything by the time you arrive. (future perfect)

I will have been traveling for three weeks by the time I return. (future perfect progressive)

Be is used to form all the progressive tenses, the tenses that are used to refer to an action or state that is continuing to happen:

I am leaving soon. (present progressive)

I was making a few calls first. (imperfect tense, also called past progressive)

I will be driving across much of the country. (future progressive)

I will have been traveling for three weeks by the time I return. (future perfect progressive)

Be is also used in the passive voice, and to join infinitives plus to to the subject of a sentence or clause:

My route was recommended by a friend. (passive voice)

I am to arrive before nightfall.

Do occurs as an auxiliary verb mainly in negative sentences and in questions:

I do not have a map.

Do you have a map handy?

Do is also occasionally used for emphasis. In this use, do is followed by an infinitive without to:

I do think I'll be able to find my way without a problem.

Do may also substitute for another verb that has just been stated:

My dog enjoyed the drive immensely, and so did I.

A small group of auxiliary verbs, called the modal verbs (or modal auxiliary verbs, modal auxiliaries, or simply modals) are only used in combination with ordinary verbs. A modal verb changes the other verb's meaning to something different from simple fact. Modals may express permission, ability, prediction, possibility, or necessity.

The principal modal verbs are: can, could, may, might, must, ought, shall, should, will, and would.

The modal verbs are different from ordinary verbs in several ways:

1) They have no inflections at all; that is, they lack an -ing form, an -ed form, and even an -s form for the third-person singular.

2) A modal verb is always followed by the infinitive form of a verb (unless that verb has already been stated) but never follows another verb.

3) Modal verbs do not follow to and are not followed by to. (Ought to, like the near-modal verb have to, is a special case.)

In their simple form, modal verbs normally refer to present or future time:

I must be nearly there by now.

I might arrive a bit later than I'd anticipated.

A trip like this can take many hours more than one expects.

Examples of auxiliary in a Sentence

Adjective Travelers have also been griping about a lack of air-conditioning while planes are on the tarmac—even after delays stretch for hours. Airlines blame the problem on a breakdown of a plane's auxiliary power unit. Darren Everson, Wall Street Journal, 8 Aug. 2007
However, on March 1, 1896, Italian forces underwent a stunning defeat at Adwa, where they fought against the warriors of the Abyssinian emperor, Menelik. Some five thousand Italian troops and two thousand native auxiliary troops had been killed or captured, and the colony had been lost. Mirella Tenderini et al., The Duke of the Abruzzi, 1997
The rotund 6'1", 235-pound object of their affections hasn't played a day of major league baseball, he dresses in the auxiliary locker room, and he isn't even listed on Boston's 40-man roster. But after a resplendent summer last year at Triple A Pawtucket and a winter of panegyrics from a giddy Boston press, first baseman Maurice Samuel (Mo) Vaughn, 23, has arrived as a "real" rookie who everyone in Red Sox-land hopes is the real thing. Nicholas Dawidoff, Sports Illustrated, 1 Apr. 1991
a sailboat with an auxiliary engine the auditorium has an auxiliary cooling system used only on particularly sweltering days Noun More than 2,000 Guard members had already been activated to help with the coronavirus pandemic, Shiroma said, but they are being moved to help law enforcement and replaced by members of the California State Guard force, a volunteer auxiliary. Don Thompson,, 2 June 2020
Roman cavalry was poorly developed; they relied on wings, called alae, of horsemen supplied by their auxiliaries. The favorite Roman battle formation was of a triple line of cohorts with alae sweeping the flanks. Kenneth P. Czech, MHQ : The Quarterly Journal of Military History, Autumn 1992
In bigger archdioceses, the archbishop cannot make long visits at many parishes; the auxiliaries will have to bear the brunt of most parish visitations. Thomas J. Reese, Archbishop, 1989
Recent Examples on the Web
Paul McCartney, the Rolling Stones, Elton John and many others in the 70-and-above group are able to deliver stadium-sized sets thanks to low-key but essential support from auxiliary players and singers — Springsteen’s current tour band totals 18 musicians. Jem Aswad, Variety, 16 May 2024 The main strategies include boosting powertrain efficiency, reducing aerodynamic drag and rolling resistance, improving the efficiency of auxiliary equipment used to heat and cool the cabin, and reducing vehicle mass. Peter Douglas, The Mercury News, 12 May 2024 See all Example Sentences for auxiliary 

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'auxiliary.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History



Middle English, "(of a Roman soldier) not belonging to a regular legion," borrowed from Latin auxiliārius "giving aid, (of troops) supplementing the regular army," from auxilium "assistance, aid, supporting force, reinforcement" (derivative from the base of augēre "to increase") + -ārius -ary entry 1 — more at eke entry 2

Note: Though the etymological literature agrees that auxilium is somehow derived from augēre—the original sense having most likely been "augmentation, reinforcement"—the formation is unclear, both in regard to the nature of the interpolated -s- and the suffix -ili-. Ernout and Meillet (Dictionnaire étymologique de la langue latine) allude to a suggestion of P. Kretschmer (Glotta, 6. Band [1914], 1. Heft, pp. 30-33), that the plural auxilia "auxiliary contingents attached to Roman armies" was originally plural of an adjective *auxilis, formed from *aux- and the suffix -ilis, "tending to, capable of" (as in docilis "teachable," facilis "easy, accommodating"; see docile, facile). The attested singular auxilium would hence be a back-formation from auxilia. The -s- is dealt with by simply taking aux- as a by-form of aug- and in effect the weak form of a noun *augos cognate with Sanskrit ójas- "strength."


derivative of auxiliary entry 1

First Known Use


15th century, in the meaning defined at sense 1a


1567, in the meaning defined at sense 1a

Time Traveler
The first known use of auxiliary was in the 15th century


Dictionary Entries Near auxiliary

Cite this Entry

“Auxiliary.” Dictionary, Merriam-Webster, Accessed 20 May. 2024.

Kids Definition


1 of 2 adjective
aux·​il·​ia·​ry ȯg-ˈzil-yə-rē How to pronounce auxiliary (audio) -ˈzil-(ə-)rē How to pronounce auxiliary (audio)
: available to provide something extra when needed
auxiliary police


2 of 2 noun
plural auxiliaries
: an auxiliary person, group, or device

Medical Definition


1 of 2 adjective
aux·​il·​ia·​ry ȯg-ˈzil-yə-rē How to pronounce auxiliary (audio) -ˈzil-(ə-)rē How to pronounce auxiliary (audio)
: serving to supplement or assist
auxiliary springs in a dental appliance


2 of 2 noun
: one who assists or serves another person especially in dentistry
: an organization that assists (as by donations or volunteer services) the work especially of a hospital

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