elementary subatomic particle similar to the electron but approximately 3,500 times heavier. Like the electron and the muon, the tau is an electrically charged member of the lepton family of subatomic particles; the tau is negatively charged, while its antiparticle is positively charged.

Being so massive, the tau is unstable, with a mean life of 2.9 × 10⁻¹³ second, and it decays readily via the weak force into other particles. The tau, like the electron and the muon, is associated with a corresponding neutral lepton, a tau- neutrino, that is produced in any decay reaction of a tau particle.

The tau was discovered through observations of its decay to muons and to electrons in the mid-1970s by a group led by Martin Perl (Perl, Martin Lewis) at the Stanford Linear Accelerator Center (SLAC) in California. Perl named the new particle, the third charged lepton, after the Greek letter that begins the word third. In 2000 scientists at the Fermi National Accelerator Laboratory reported the first experimental evidence for the existence of the tau-neutrino, the tau's elusive partner.