Full Idea
Jespersen calls a noun a mass word when it has no plural, does not take numerical adjectives, and does not take 'fewer'.
Gist of Idea
Mass words do not have plurals, or numerical adjectives, or use 'fewer'
Source
William D. Hart (The Evolution of Logic [2010], 3)
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.71
A Reaction
Jespersen was a great linguistics expert.