A noun that does not form plurals WordNet 3.0 & Open English WordNet

In linguistics, a mass noun, uncountable noun, non-count noun, uncount noun, or just uncountable, is a noun with the syntactic property that any quantity of it is treated as an undifferentiated unit, rather than as something with discrete elements. Wikipedia

Noun or noun phrase whose quantity is indiscrete and has no innate singularity or plurality. Wikidata

A noun that normally cannot be counted and thus does not form plurals and presents entities as an indivisible mass. OmegaWiki

A noun that normally cannot be counted. Wiktionary

Noun describing something that cannot be counted. Wiktionary (translation)