An

• POS: Conjunction, Definite Article, Noun

CIDE DICTIONARY

An,  [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.  [1913 Webster]
" 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."  [1913 Webster]

An, conj. [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.  [1913 Webster]
"Nay, an thou dalliest, then I am thy foe."  [1913 Webster]

An if, and if; if.

OXFORD DICTIONARY

An, adj. the form of the indefinite article (see A(1)) used before words beginning with a vowel sound (an egg; an hour; an MP).

Usage

Now less often used before aspirated words beginning with h and stressed on a syllable other than the first (so a hotel, not an hotel).

ROGET THESAURUS

An

Supposition

N supposition, assumption, assumed position, postulation, condition, presupposition, hypothesis, blue sky hypothesis, postulate, postulatum, theory, thesis, theorem, data, proposition, position, proposal, presumption, divination, conjecture, guess, guesswork, speculation, rough guess, shot, shot in the dark, conjecturality, surmise, suspicion, sneaking suspicion, estimate, approximation (nearness), inkling, suggestion, hint, intimation, notion, impression, bare supposition, vague supposition, loose supposition, loose suggestion, association of ideas, (analogy), metonym, metonymy, simile (metaphor), conceit, idea, thought, original idea, invention (imagination), supposing, given, mooted, postulatory, assumed, supposititious, suppositive, suppositious, gratuitous, speculative, conjectural, hypothetical, theoretical, academic, supposable, presumptive, putative, suppositional, suggestive, allusive, if, if so be, an, on the supposition, ex hypothesi, in the case, in the event of, quasi, as if, provided, perhaps, for aught one knows.