tau

Etymology

From Middle English tau, taue, from Latin tau, from Ancient Greek ταῦ (taû) and Hebrew תָּו (tav). Sense 6 (“mathematical constant equal to 2π”) was used by Joseph M. Lindenberg in 1991, and popularized by the American educator and entrepreneur Michael Hartl in a 2010 paper which explains that τ resembles π; and that τ is the Greek equivalent of t, the first letter of turn, and 2π corresponds to one turn of a circle with a radius of one unit. Sense 8.1 (“short for tau lepton or tau particle”) was coined by the American physicist Martin Lewis Perl (1927–2014) after the first letter of Ancient Greek τρίτον (tríton, “third”), since the tau lepton or tauon was the third charged lepton discovered.

noun

1. The letter Τ／τ in the Greek alphabet; being the nineteenth letter of the Classical and Modern Greek, and the twenty-first letter of the Old and Ancient Greek alphabets.
2. Alternative form of taw; the 22nd and last letter of many Semitic alphabets/abjads, including Phoenician, Aramaic, Hebrew, Syriac, and Arabic.
   Hence it appears that the spits, or skewers, on which and to which the lamb was fixed and fastened in order to be roasted, assumed the form of a cross, not such a tau-cross as is engraved in Dr. Oliver’s Historical Landmarks of Freemasonry, vol. i. p. 80. having three arms only like the Greek letter tau; but a cross like the ancient Hebrew tau, with four arms, though not necessarily all of equal length. 1847, Richard Edmund Tyrwhitt, Sermons Chiefly Expository, volume I, Oxford: John Henry Parker; F[rancis] and J[ohn] Rivington, London, page 366
   In the Spanish translation of Sallust, by the Infant Don Gabriel in 1772, called the Infant Sallust, there is a curious dissertation by Father Perez Bayer on the resemblance between the ancient Hebrew and Phœnician alphabets, in which it is observed that the Hebrew Tau was written in pure Phœnician, […] 1851, D[aniel] Rock, Hierurgia; or Transubstantiation, Invocation of Saints, Relics, and Purgatory, Besides Those Other Articles of Doctrine Set Forth in the Holy Sacrifice of the Mass, Expounded;[…], 2nd edition, London: C. Dolman,[…], page 350
   The tau is both the 19ᵗʰ letter of the Greek alphabet, and also the 22ⁿᵈ letter of the Hebrew alphabet. In this context, the Hebrew tau or tav is more pertinent. 2017, Piers Vaughan, Capitular Development Course, 2nd edition, Rose Circle Publications, page 135
3. A Τ-shaped object or sign; a Saint Anthony's cross, sometimes regarded as a sacred symbol.
   1. (Christianity) A crosier with a Τ-shaped head.
4. The ankh symbol (☥).
5. (astronomy) Chiefly written τ: used to designate the nineteenth star (usually according to brightness) in a constellation.
6. (finance) A measurement of the sensitivity of the value of an option to changes in the implied volatility of the price of the underlying asset.
7. (mathematics, neologism) Chiefly written τ: an irrational and transcendental constant representing the ratio of the circumference of a Euclidean circle to its radius, equal to twice the value of pi (2π; approximately 6.2831853071).
8. (neurology) Short for tau protein (“a protein abundant especially in the neurons of the human central nervous system that stabilizes microtubules, and when misfolded is associated with forms of dementia such as Alzheimer's and Parkinson's diseases”).
   Quite what that job is remains obscure, but one theory is that it is to stabilise another protein called tau, which is supposed in turn to keep in shape the tubular ‘skeleton’ of a neuron. 1999, Matt Ridley, Genome: The Autobiography of a Species in 23 Chapters, London: Harper Perennial, published 2004, page 263
9. (physics) Chiefly written τ.
   1. Short for tau lepton or tau particle (“an unstable elementary particle which is a type of lepton, having a mass almost twice that of a proton, a negative charge, and a spin of ½; it decays into hadrons (usually pions) or other leptons, and neutrinos; a tauon”).
   2. (historical) Short for tau meson, now known as a kaon.