Etymology
The name lepton comes from the Greek "λεπτόν" (lepton), neuter of "λεπτός" (leptos), "fine, small, thin" and the earliest attested form of the word is the Mycenaean Greek 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.
The etymology incorrectly implies that all the leptons are of small mass. When Rosenfeld named them, the only known leptons were electrons and muons, which are in fact of small mass — the mass of an electron (0.511 MeV/c2) and the mass of a muon (with a value of 105.7 MeV/c2) are fractions of the mass of the "heavy" proton (938.3 MeV/c2). However, the mass of the tau (discovered in the mid 1970s) (1,777 MeV/c2) is nearly twice that of the proton, and about 3,500 times that of the electron.
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