A neutrino is a nearly massless subatomic particle that is part of the lepton family. It is an electrically neutral particle that interacts only weakly with other particles, making it difficult to detect. Neutrinos are produced in nuclear reactions and radioactive decay, and they can travel through matter almost undisturbed. They are important in astrophysics, as they are believed to be responsible for the majority of the energy released in supernovae.