tau

noun

1. 1

   The letter Τ/τ in the Greek, Hebrew and ancient Semitic alphabets, being the nineteenth letter of the Classical and Modern Greek, the twenty-first letter of Old and Ancient Greek.

2. 2

   A Τ-shaped sign or structure; a Saint Anthony's cross, sometimes regarded as a sacred symbol.

   “1658: Nor shall we take in the mysticall Tau, or the Crosse of our blessed Saviour, which having in some descriptions an Empedon or crossing foot-stay, made not one single transversion. — Sir Thomas Browne, The Garden of Cyrus (Folio Society 2007, p. 168)”

3. 3

   A tau meson, now usually known as a kaon.

4. 4

   An unstable heavy lepton, which decays into a muon or electron; a tauon.

5. 5

   Tau protein, a type of protein that stabilizes microtubules in the human central nervous system, failures of which are associated with forms of dementia.

6. 6

   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. 7

   An irrational and transcendental constant representing the ratio of the circumference of a Euclidean circle to its radius, approximately 6.2831853071; equal to twice the value of pi. Often written τ or 2π.