In linguistics, a mass noun is a common noun that refers to an amount, not an item or unit. Common mass nouns are water, chocolate, meat, time, and space. Mass nouns are always singular and uncountable. The set of uncountable nouns and mass nouns is not identical. There are uncountable nouns, such as scissors which refer to an item, are plural, and are not mass nouns.
In some languages like English, mass nouns have different grammar than countable nouns. Their requirements for determiners, for example differ. Other languages, such as Japanese, do not treat mass nouns in any special way.