Online English Dictionary

A (noun) - symbol, abbreviation "noun"

indefinite article The form "a" is used before consonant sounds and the form an before vowel sounds. When saying abbreviations like ‘FM’ or ‘UN’, use a or an according to how the first letter is said. For example, F is a consonant, but begins with the sound / e / and so you say:an FM radio. U is a vowel but begins with / j / and so you say: a UN declaration.1used before countable or singular nouns referring to people or things that have not already been mentioned:
a man / horse / unit Ç an aunt / egg / hour / x-ray