Толковый словарь английского языка Oxford English Reference - 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). AN suffix (also -ean, -ian) forming adjectives and nouns, esp. from names of places, systems, zoological classes or orders, and founders (Mexican; Anglican; crustacean; European; Lutheran; Georgian; theologian). Etymology: ult. f. L adj. endings -(i)anus, -aeus: cf. Gk -aios, -eios