In linguistics, a mass noun (also uncountable 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 litres 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.