A
neutrino is a neutral particle with very low
mass, possibly zero. It has spin 1/2 and so is a
fermion. It does not interact with the
strong force or the
electromagnetic force, but does interact with the
weak force (and with
gravity if it turns out to have mass).
It comes in three varieties, the electron neutrino νe, the muon neutrino νμ, and the tau neutrino ντ.
See also solar neutrino problem, particle physics.