Any of a class of six elementary fermions, two in each of the three generations of fundamental fermions (one negatively charged and one neutral), and including the electron, electron neutrino, muon, muon neutrino, tauon, and tauon neutrino, together with their associated antiparticles, the antileptons. Leptons participate in weak interactions, but not strong interactions, and have masses generally less than those of mesons and baryons.