information:highlighting, linguistic:paraphrasing
Etymology

The name lepton comes from the Greek λεπτός leptós, "fine, small, thin" (neuter nominative/accusative singular form: λεπτόν leptón); the earliest attested form of the word is the Mycenaean Greek 𐀩𐀡𐀵, re-po-to, written in Linear B syllabic script. Lepton was first used by physicist Léon Rosenfeld in 1948:

Following a suggestion of Prof. C. Møller, I adopt—as a pendant to "nucleon"—the denomination "lepton" (from λεπτός, small, thin, delicate) to denote a particle of small mass.

Rosenfeld chose the name as the common name for electrons and (then hypothesized) neutrinos. Additionally, the muon, initially classified as a meson, was reclassified as a lepton in the 1950s. The masses of those particles are small compared to nucleons—the mass of an electron (0.511 MeV/ c) and the mass of a muon (with a value of 105.7 MeV/ c) are fractions of the mass of the "heavy" proton (938.3 MeV/ c), and the mass of a neutrino is nearly zero. However, the mass of the tau (discovered in the mid-1970s) (1777 MeV/ c) is nearly twice that of the proton and 3477 times that of the electron.