(astronomy) Chiefly written τ: used to designate the nineteenth star (usually according to brightness) in a constellation.
(historical) Short for tau meson, now known as a kaon.
(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”).
A Τ-shaped object or sign; a Saint Anthony's cross, sometimes regarded as a sacred symbol.
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”).
The ankh symbol (☥).
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.
