The words a and an in English. Indefinite articles are used to introduce a noun as a member of a class of nouns, or to make generalizations about a whole class or group of nouns e.g. Please hand me pencil. An ocean is large body of water.
The indefinite articles are and an. They are used to express a general thing. is used before singular countable nouns that begin with consonants. For example: book. An is used before singular countable nouns that begin with vowels: For example: An umbrella. or vowel-like sounds. For example: An X-ray.