A noun referring to something which is considered to be indeterminate and unquantifiable in number, handled grammatically using singular forms regardless of the quantity; plural forms may exist but involve a change in semantic value above and beyond mere plurality. Contrast count noun. Synonyms: uncountable noun, non-count noun.
Most European languages treat substances as mass nouns; here plural forms exist but refer only to different types (a spoonful of sugar/glucose and sucrose are sugars), discrete objects (rocks) versus a literal mass (rock), or more complex distinctions (water versus waters). English makes more use of this form than, notably, the Romance languages, however: notionally countable items like hair are also treated as uncountable (contrast FR les cheveux, IT i capelli for instance; note that unlike, say, "sand" the word can also refer to a discrete individual piece) as well as a range of more abstract concepts like "information" (cf FR les informations); in this context the much-disputed handling of "data" as an inflectionally singular noun makes reasonable sense - this can be seen as the full adoption of a Latin-rooted lexeme into a language with predominantly Germanic structures, particularly if "data" is considered to refer to a broader range of meanings than mere numbers (a reductionistic view which may have been encouraged by the digitizing of everything; nonetheless it is hard to argue that the simple possibility of storing the words of A la recherche du temps perdu in ASCII or the recording of The Birdie Song as an MP3 means that all the data incorporated in these works is countable.
This is an issue of some significance for language learners; note also that British and American English usage differs for some items: BE treats "accommodation" (in the sense of a roof over your head) as uncountable, while Americans look for accommodations.