The articles ‘an’ and ‘an’ are termed as indefinite articles. An indefinite article, as the name suggests, is used to indicate something that is not definite or specific. It can also be used before singular nouns. Among the indefinite articles, ‘an’ is used before singular nouns that start with vowel sounds and ‘a’ is used before singular nouns that begin with consonant sounds.
For example:
1. I had an apple for breakfast.
2. Do you have an eraser?
3. I saw an aeroplane.
4. She has a pet dog.