Tau

noun

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. 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. A tau meson, now usually known as a kaon.
4. An unstable heavy lepton, which decays into a muon or electron; a tauon.
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.