A and an function primarily as the indefinite forms of the grammatical article in the English language.

An is the older form, now used before words starting with a vowel sound. It is also used before words beginning with an H sound in an unstressed syllable as in an historic event. This second usage is fading in written English and some dialects of spoken English, such as American English, where it is more common to use "a" rather than "an" before a pronounced H.

Indeed, the N has wandered back and forth between words beginning with vowels over the history of the language, where sometimes it would be a nuncle and is now an uncle. The Oxford English Dictionary gives such examples as smot hym on the hede with a nege tool from 1448 for smote him on the head with an edge tool and a nox for an ox and a napple for an apple. Sometimes the change has been permanent. For example, a newt was once an ewt (earlier euft and eft), a nickname was once an eke-name, where eke means "extra" (as in eke out meaning "add to"), and in the other direction, an orange was once a norange.

In addition to serving as an article, a and an are also used as synonyms for the number one, as in "make a wish", "a hundred". An was originally an unstressed form of the number án 'one'.

A and an are also used to express a proportional relationship, such as "a dollar a day" or "$50 an ounce", although historically this use of "a" and "an" does not come from the same word as the articles.

See also the.