what is tau
Definition of TAU. 1. : the 19th letter of the Greek alphabet â see alphabet table.2. : a short-lived elementary particle of the lepton family that exists in positive and negative charge states and has a mass about 3500 times greater than an electron âcalled also tau particle.3. the 19th letter of the Greek alphabet â see alphabet table. 2. : a short-lived elementary particle of the lepton family that exists in positive and negative charge states and has a mass about 3500 times greater than an electron âcalled also tau particle. 3.