tau lepton

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The flavor of lepton that belongs to the third generation of elementary fermions and has a negative electric charge, a mass 3.477 times that of an electron, and a mean lifetime of about 2.9 × 10⁻¹³ second.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics A tauon.