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. • Alternative form of taw; the 22nd and last letter of many Semitic alphabets/abjads, including Phoenician, Aramaic, Hebrew, Syriac, and Arabic. • A Τ-shaped object or sign; a Saint Anthony's cross, sometimes regarded as a sacred symbol. • A crosier with a Τ-shaped head. • The ankh symbol (☥). • Chiefly written τ: used to designate the nineteenth star (usually according to brightness) in a constellation. • A measurement of the sensitivity of the value of an option to changes in the implied volatility of the price of the underlying asset. • 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). • Ellipsis of 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”). • Chiefly written τ. • Ellipsis of 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”). • Ellipsis of tau meson, now known as a kaon.