In linguistics, a mass noun (also uncountable noun or non-count noun) is a noun with the syntactic property that any quantity of it is treated as an undifferentiated unit, rather than as something with discrete subsets. Non-count nouns are distinguished from count nouns.

Given that different languages have different grammatical features, the actual test for which nouns are mass nouns may vary between languages. In English, mass nouns are characterized by the fact that they cannot be directly modified by a numeral without specifying a unit of measurement, and that they cannot combine with an indefinite article (a or an). Thus, the mass noun "water" is quantified as "20 liters of water" while the count noun "chair" is quantified as "20 chairs". However, mass nouns (like count nouns) can be quantified in relative terms without unit specification (e.g., "much water," "many chairs").

Some mass nouns can be used in English in the plural to mean "more than one instance (or example) of a certain sort of entity"—for example, "Many cleaning agents today are technically not soaps, but detergents." In such cases they no longer play the role of mass nouns, but (syntactically) they are treated as count nouns.

In physics, mass (from Greek μᾶζα "barley cake, lump (of dough)"), more specifically inertial mass, can be defined as a quantitative measure of an object's resistance to the change of its speed. In addition to this, gravitational mass can be described as a measure of magnitude of the gravitational force which is

when interacting with a second object. The SI unit of mass is the kilogram (kg).

In everyday usage, mass is often referred to as weight, the units of which are often taken to be kilograms (for instance, a person may state that their weight is 75 kg). In scientific use, however, the term weight refers to a different, yet related, property of matter. Weight is the gravitational force acting on a given body—which differs depending on the gravitational pull of the opposing body (e.g. a person's weight on Earth vs on the Moon) — while mass is an intrinsic property of that body that never changes. In other words, an object's weight depends on its environment, while its mass does not. On the surface of the Earth, an object with a mass of 50 kilograms weighs 491 Newtons; on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 Newtons. Restated in mathematical terms, on the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g is the Earth's gravitational field strength, equal to about 9.81 m/s².

Put simply, a noun is a word used to name a person, place, or thing,.

In linguistics, a noun is a member of a large, open lexical category whose members can occur as the main word in the subject of a clause, the object of a verb, or the object of a preposition.

Lexical categories are defined in terms of how their members combine with other kinds of expressions. The syntactic rules for nouns differ from language to language. In English, nouns may be defined as those words which can occur with articles and attributive adjectives and can function as the head of a noun phrase.

In traditional English grammar, the noun is one of the eight parts of speech.

Noun comes from the Latin nōmen "name", a translation of Ancient Greek ónoma. Word classes like nouns were first described by Pāṇini in the Sanskrit language and by Ancient Greek grammarians, and were defined by the grammatical forms that they take. In Greek and Sanskrit, for example, nouns are categorized by gender and inflected for case and number.