In linguistics, a mass noun, uncountable noun, non-count noun, uncount noun, or just uncountable, is a noun with the syntactic property that any quantity of it is treated as an undifferentiated unit, rather than as something with discrete elements. 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 impossibility of being directly modified by a numeral without specifying a unit of measurement and by the impossibility of being combined 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, both mass and count nouns can be quantified in relative terms without unit specification (e.g., "so much water", "so many chairs").
Mass nouns have no concept of singular and plural, although in English they take singular verb forms. However, many mass nouns in English can be converted to count nouns, which can then be used in the plural to denote (for instance) more than one instance or variety of a certain sort of entity – for example, "Many cleaning agents today are technically not soaps [i.e. types of soap], but detergents," or "I drank about three beers [i.e. bottles or glasses of beer]".
Some nouns can be used indifferently as mass or count nouns, e.g., three cabbages or three heads of cabbage; three ropes or three lengths of rope. Some have different senses as mass and count nouns: paper is a mass noun as a material (three reams of paper, one sheet of paper), but a count noun as a unit of writing ("the students passed in their papers").