сокращение от thousand astronomical units тысяча астрономических единиц
noun Etymology: Middle English taw, from Latin tau, from Greek, of Semitic origin; akin to Hebrew tāw taw 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 tau 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
The mathematical constant that is the circumference of a circle divided by its radius, equal to twice pi. Some have suggested that pi is the wrong choice of constant to describe the geometry of circles etc. and tau = 2*pi would have been better. Most practising mathematicians think this is silly.