neutrino

Definition: a lepton that exists in three forms, one for each mass generation, believed to have a very small, but nonzero mass, typically moving at very high speed. Neutrinos are very weakly interacting, of the billions passing through you from the sun, only a very small number will interact with the atoms in your body at all.

The neutrino was predicted in 1931 to account for the "missing energy" in beta decay, and was identified in 1953.

The name comes from Enrico Fermi, and means little neutral one in his native Italian.