In grammar, a noun is a word that represents a concrete or abstract thing, like living creatures, places, actions, qualities, states of existence, and ideas. A noun may serve as an object or subject within a phrase, clause, or sentence.

In linguistics, nouns constitute a lexical category (part of speech) defined according to how its members combine with members of other lexical categories. The syntactic occurrence of nouns differs among languages.

In English, prototypical nouns are common nouns or proper nouns that can occur with determiners, articles and attributive adjectives, and can function as the head of a noun phrase. According to traditional and popular classification, pronouns are distinct from nouns, but in much modern theory they are considered a subclass of nouns. Every language has various linguistic and grammatical distinctions between nouns and verbs.

Word classes (parts of speech) were described by Sanskrit grammarians from at least the 5th century BC. In Yāska's Nirukta, the noun (nāma) is one of the four main categories of words defined.

The Ancient Greek equivalent was ónoma (ὄνομα), referred to by Plato in the Cratylus dialog, and later listed as one of the eight parts of speech in The Art of Grammar, attributed to Dionysius Thrax (2nd century BC). The term used in Latin grammar was nōmen. All of these terms for "noun" were also words meaning "name". The English word noun is derived from the Latin term, through the Anglo-Norman nom (other forms include nomme, and noun itself).

The word classes were defined partly by the grammatical forms that they take. In Sanskrit, Greek, and Latin, for example, nouns are categorized by gender and inflected for case and number. Because adjectives share these three grammatical categories, adjectives typically were placed in the same class as nouns.

Similarly, the Latin term nōmen includes both nouns (substantives) and adjectives, as originally did the English word noun, the two types being distinguished as nouns substantive and nouns adjective (or substantive nouns and adjective nouns, or simply substantives and adjectives). (The word nominal is now sometimes used to denote a class that includes both nouns and adjectives.)

Many European languages use a cognate of the word substantive as the basic term for noun (for example, Spanish sustantivo, "noun"). Nouns in the dictionaries of such languages are demarked by the abbreviation s. or sb. instead of n., which may be used for proper nouns or neuter nouns instead. In English, some modern authors use the word substantive to refer to a class that includes both nouns (single words) and noun phrases (multiword units that are sometimes called noun equivalents). It can also be used as a counterpart to attributive when distinguishing between a noun being used as the head (main word) of a noun phrase and a noun being used as a noun adjunct. For example, the noun knee can be said to be used substantively in my knee hurts, but attributively in the patient needed knee replacement.