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.