Lepton, one of the 4 classes of elementary particles (the others are bosons, measons, and baryons). Leptons are larger than the massless bosons, but smaller than mesons and baryons. There are a total of 12 particles in the lepton class, of which the electron is probably the most familiar. These 12 come in pairs, one of each pair being positively charged, the other being negatively charged. (The positron is the positively charged mate of the electron, which is negative.)
See also: Particle physics.