GNU Collaborative International Dictionary of English

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Found one definition

1.       
   lepton n. (Physics) an elementary particle that participates in weak interactions but does not participate in the strong interaction; it has a baryon number of 0.  Some known leptons are the electron, the negative muon, the tau-minus particle, and the neutrinos associated with each of these particles.
   
   [WordNet 1.5 +PJC]