Any of three species of very weakly-interacting lepton with an extremely small, possibly zero, mass. Electron neutrinos are generated in the interior of the Sun (and other stars) in nuclear reactions. Generally such neutrinos do not interact with matter and stream out through the Sun. A very few of these many neutrinos can be detected in sophisticated detectors here on Earth, giving us a "window" into the interior of the Sun. In 1987 neutrinos from a Supernova in the Large Magellanic Cloud were detected in terrestrial neutrino experiments.
If neutrinos have a small but nonzero mass, they would constitute an important type of dark matter in the universe.