Definition 0.1   A lepton is a spin 1/2 particle fermion that does not interact via the strong force. To date there are three known types of leptons. They are: the electron, the Muon (), and the Tau () with their corresponding anti–particles that carry oposite charge ( instead of ). Neutrinos are also leptons and are divided in three categories that follow their massive partners, .

All leptons (with the notable exceptions of all Neutrino types) have electrical charge, and their masses are well established. Also the number of leptons is conserved in interactions and nuclear reactions even though the mass of the system might change. Flavour on the other hand is not conserved like in the case of neutrino oscillations or muon decay.