An

An is the English indefinite article used before a non-specific, singular noun to indicate one member of a class rather than a particular item. It helps introduce new information and contrasts with the definite article, the.

Etymology: The form derives from Old English ān, meaning “one” or “single,” and is related to Proto-Germanic *ainaz. Over time, the language settled on two spellings, a and an, chosen to align with the initial sound of the following word.

Usage: Use an before words that begin with a vowel sound. This includes words starting with a vowel letter and those whose initial sound is a vowel, as well as words with a silent or reduced consonant. Examples: an apple, an hour, an honest mistake, an umbrella. Use a before words that begin with a consonant sound, such as a cat, a university, or a European country before the initial consonant sound. The article is not used with plural nouns or noncount nouns: you would say an apple, but not an apples or an advice.

Special cases: With acronyms and initialisms, the choice follows pronunciation rather than spelling. For example, an FBI agent (the initial sound is a vowel), but a NASA mission (NASA is pronounced as a word beginning with a consonant). Numbers also follow the rule: an 18-year-old, an 8-ounce serving. Historically the distinction was broader, but modern usage emphasizes the sound that begins the following word.

See also: The indefinite article contrasts with the definite article the, and with zero article in certain plural or uncountable contexts.