Meaning of AN in English
■ determiner the form of the indefinite article (see a 1 ) used before words beginning with a vowel sound.
It is better to use a rather th~ ~ before words such as hotel ~d historical that begin with a sounded h- . An was common in the 18th ~d 19th centuries because the initial h was then often not pronounced.
Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004