- (physics, dated) A tau meson, now usually known as a kaon.
- (mathematics, neologism) A constant equal to twice the value of pi; often written τ.
- A Τ-shaped sign or structure; a St. Anthony's cross, sometimes considered as a sacred symbol.
- (biology) A type of protein that stabilizes microtubules in the human central nervous system, failures of which are associated with forms of dementia.
- The name of 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.
- (physics) An unstable heavy lepton, which decays into a muon or electron; a tauon.