Definition - "Jespersen's term for words denoting concepts, properties or things which ordinarily cannot be separated into distinct component units."
(Source: Pei & Gaynor, Dictionary of Linguistics (1954), s.v. mass-word)

Example -
silver, water, butter, leisure, music, traffic, and progress

Etymology -
The term was coined by Otto Jesperson (c. 1914) for use in his book Modern English Grammar on Historical Principles (see OED citation below).

Oxford English Dictionary -
Its first citation is from 1914:
"Words which represent ‘uncountables’ … are here called mass-words; they may be either material … such as silver, quicksilver, water, butter,… or else immaterial, such as leisure, music, traffic, progress, [etc.]."
(O. Jespersen Mod. Eng. Gram. II. v. 115)

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