Meaning of AN in English

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). °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.