tau

Tau is the name of the nineteenth letter of the Greek alphabet. Its uppercase form is Τ and lowercase is τ. The letter derives from the Phoenician taw and is used in Greek to represent the voiceless 't' sound. In modern use, it appears in various scientific, mathematical, and linguistic contexts as a symbol.

In science and mathematics, tau denotes several quantities: torque in mechanics, where τ represents the moment of force; the time constant in many exponential processes such as RC circuits, where voltage or current decays with time constant τ; and in particle physics, the tau lepton (a heavy charged lepton) and the tau neutrino, its neutral partner.

In geometry and number theory there is also a proposal to adopt the circle constant tau as 2π, rather than π, for calculations involving radians. Proponents argue that tau streamlines many formulas. The value is tau ≈ 6.283185307..., i.e., tau equals two times pi; however, pi remains the more widely used constant.

In biology, tau is the name of a microtubule-associated protein that stabilizes microtubules in neurons. Abnormal tau proteins can aggregate into neurofibrillary tangles and are implicated in neurodegenerative diseases such as Alzheimer's disease and other tauopathies.