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A lepton is a class of elementary particle that includes electrons, specifically an elementary particle that is a fermion (i.e., with 1/2 integer spin) but does not respond to the strong force (as do quarks). The standard model presumes the six known types of leptons (termed lepton flavors). Three of these lepton types have electric charges: the electron, the muon, and the tau, and the term neutrino refers to any of the other three, that are neutral: the electron neutrino, the muon neutrino, and the tau neutrino. These neutrino type names reflect that fact that for each, there exists a particle interaction (a type of weak interaction) between that type of neutrino and the associated charged lepton. Each of these six has an associated antiparticle, but currently, a hypothesis of interest is that the neutrino types could be their own antiparticles, i.e., Majorana particles.