an1. (ăn). [AS. ān one, the same word as the numeral. See One, and cf. A.] This word is properly an adjective, but is commonly called the indefinite article. It is used before nouns of the singular number only, and signifies one, or any, but somewhat less emphatically. In such expressions as “twice an hour,” “once an age,” a shilling an ounce (see 2d A, 2), it has a distributive force, and is equivalent to each, every.

☞ An is used before a word beginning with a vowel sound; as, an enemy, an hour. It in also often used before h sounded, when the accent of the word falls on the second syllable; as, an historian, an hyena, an heroic deed. Many writers use a before h in such positions. Anciently an was used before consonants as well as vowels.

2. conjunction [Shortened fr. and, OE. an., and, sometimes and if, in introducing conditional clauses, like Icel. enda if, the same word as and. Prob. and was originally pleonastic before the conditional clause.]

If; – a word used by old English authors. Shak.

Nay, an thou dalliest, then I am thy foe. B. Jonson.

An if and if; if.

-- Webster's unabridged 1913