Meaning of AN in English
adj. the form of the indefinite article (see A(1)) used before words beginning with a vowel sound (an egg; an hour; an MP). °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).
Concise Oxford English dictionary. Краткий оксфордский словарь английского языка. 2012