A noun is a word used to name a person, animal, place, thing, and abstract idea. Nouns are usually the first words which small children learn. The highlighted words in the following sentences are all nouns:

Late last year our neighbours bought a goat.

Portia White was an opera singer.

The bus inspector looked at all the passengers' passes.

According to Plutarch, the library at Alexandria was destroyed in 48 B.C.

Philosophy is of little comfort to the starving.

A noun can function in a sentence as a subject, a direct object, an indirect object, a subject complement, an object complement, an appositive, an adjective or an adverb.