A mass noun is a type of noun not typically quantified into individual units and pluralized, but can refer to one or more things that are not actually counted. The word “water,” for example, is such a noun since it can be used to refer to a particular thing, but is not typically pluralized as “waters” and an indefinite article such as “a” or “an” is not used before it. This type of noun can also be a countable noun, depending on the particular way in which it is used, and context often establishes what type of noun a particular word is in a certain usage.
Also referred to as an uncountable noun, a mass noun is much like other nouns in that it refers to a person, place, thing, or idea but does so without the ability to be quantified. “Water” is an excellent example of a mass noun, as it clearly refers to a particular substance. Unlike countable nouns like “dog,” “chair,” and “glass,” it is never pluralized with an “-s” ending such as “dogs,” “chairs,” and “glasses.” To be pluralized in such a way, some type of other noun or phrase is typically used to quantify it, such as “a glass of water.”
A mass noun also cannot typically be preceded by an indefinite article; “a dog” or “an orangutan” is acceptable, but standard English does not allow for “a water” or “an oxygen.” There are some exceptions to this in certain situations in which an uncountable noun is part of a larger idea. It is acceptable to say “an oxygen molecule” or “a glass of water,” as these refer to nouns other than the ideas represented simply by “oxygen” or “water” by itself. A mass noun is also typically used to refer to a particular subject or area of study such as “mathematics” or “photography.”
There are ways in which these nouns can be pluralized, however this typically refers to another concept that the noun is representing. Someone saying “Please get our waters from the dining room” is likely referring to “glasses of water” and simply saying “waters” to refer to them. Similarly, a mass noun like “reading” can be used as both a countable and uncountable noun depending on its context. “Your reading” refers to the general idea of a selection that someone is supposed to read, while “your three readings from yesterday” refers to three particular selections that were read. This specificity allows the noun to be counted, which means the word can be pluralized.