| Definition -
(Aka non-count noun | mass noun)
A noun that
(1) has no plural form,
(2) takes only a singular verb, and
(3) can be used without a determiner.
1. This is in contrast to countable nouns, such as book, which can be pluralized.
Example - The word furniture is an uncountable noun because:
(1) it has no plural forms (e.g. you can't buy two furnitures);
(2) it can only be used with a singular verb (you can't say that the furniture are nice); and
(3) it can be used without a determiner.