Wiktionary:Word of the day/2022/June 28

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tau n

The letter Τ/τ in the Greek, Hebrew, and ancient Semitic alphabets; being the nineteenth letter of the Classical and Modern Greek, and the twenty-first letter of the Old and Ancient Greek alphabets. A Τ-shaped object or sign; a Saint Anthony's cross, sometimes regarded as a sacred symbol. (Christianity) A crosier with a Τ-shaped head. The ankh symbol (☥). (astronomy) Chiefly written τ: used to designate the nineteenth star (usually according to brightness) in a constellation. (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. (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). (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”). (physics) Chiefly written τ. 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”). (historical) Short for tau meson, now known as a kaon. Today is Tau Day, which was invented by the American educator and entrepreneur Michael Hartl to promote the use of τ as a mathematical constant in place of 2π. June 28 is written as 6/28 in the month/day format, and 6, 2, and 8 are the first three significant digits of τ.

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