A lepton is a fundamental particle of matter that does not take part in strong interactions. All leptons are fermions and have spin ½. The charged leptons are the electron, the muon, the tau lepton, and their antiparticles. Neutral leptons are called neutrinos. Lepton number is the total number of leptons present in a system minus the total number of antileptons. Lepton conservation is a rule which states that the net number of leptons before and after an interaction must be the same. The name 'lepton' comes from Greek leptos meaning 'small' or 'slight' and was coined by L. Rosenfeld in 1948.
Lepton flavour violation
Lepton flavor violation is the principle, in the Standard Model, that leptons do not change flavor. For example, a muon would never turn into an electron. If this were observed, it would be a signal for new physics. Evidence for neutrino oscillations already indicate that lepton flavor violation may occur.