Neutrinos are fundamental particles in the lepton family of the Standard Model. Each generation of the lepton family includes a neutrino (see Unit 1, Fig. 18). Neutrinos are electrically neutral and nearly massless. When neutrinos are classified according to their lepton family generation, the three different types of neutrinos (electron, muon, and tau) are referred to as "neutrino flavors." While neutrinos are created as a well-defined flavor, the three different flavors mix together as the neutrinos travel through space, a phenomenon referred to as "flavor oscillation." Determining the exact neutrino masses and oscillation parameters is still an active area of research.