an

from The American Heritage® Dictionary of the English Language, 5th Edition.

• conjunction And if; if.
• article The form of a used before words beginning with a vowel sound.

from The Century Dictionary.

• A prefix of Anglo-Saxon origin, a reduced form of and- (which see), occurring unfelt in answer.
• noun The indefinite article.
• A suffix of Latin origin, forming adjectives which are or may be also used as nouns.
• A prefix of Anglo-Saxon origin, the same as on- and a-, occurring unfelt in anent, anon, anan, aneal, aneal, etc., and with accent in anvil (but in this and some other words perhaps originally and-: see an).
• An earlier form of on, retained until the last century in certain phrases, as an edge, an end, now only on edge, on end; in present use only as an unfelt prefix an- or reduced a-. See an-, a-.
• A prefix of Latin origin, a reduced form of ambi-, occurring (unfelt in English) in ancile, ancipital, anfractuous, etc.
• Coordinate use: And; same as and, A.
• Conditional use: If; same as and, B.
• A prefix of Greek origin, the fuller form of ἀ- privative (a-) preserved before a vowel, as in anarchy, anarthrous, anecdote, anomaly, etc.
• A prefix of Greek origin, the form of ana- before a vowel, as in anode.
• A prefix of Latin origin, usually an assimilation of ad- before n-, as in annex, annul, announce, etc., but sometimes representing Latin in-, as in anoint, annoy.

from the GNU version of the Collaborative International Dictionary of English.

• 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.
• conjunction If; -- a word used by old English authors.
• conjunction and if; if.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• conjunction archaic If, so long as.
• conjunction archaic as if; as though.
• preposition In each; to or for each; per.
• determiner, article Form, used before a vowel sound, of a
• noun The first letter of the Georgian alphabet, ა (mkhedruli), Ⴀ (asomtavruli) or ⴀ (nuskhuri).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an associate degree in nursing