Merriam-Webster Collegiate Dictionary - tau

 noun  Etymology: Middle English taw, from Latin ~, from Greek, of Semitic origin; akin to Hebrew taw taw  Date: 14th century  1. the 19th letter of the Greek alphabet — 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 — called also ~ particle  3. a protein that binds to and regulates the assembly and stability of neuronal microtubules and that is found in an abnormal form as the major component of neurofibrillary tangles