| Definition -
A noun that can't be counted, e.g. information, furniture, and money.
1. They usually can't be pluralized and are almost never used with indefinite articles.
2. This is in contrast to countable nouns, such as book, which can be pluralized.
Oxford English Dictionary - The term's first citation is from 1933:
"Mass nouns never take a and have no plural."
(L. Bloomfield Lang. xii. 205)