What is the meaning of τ?

Torque.

Shear stress.

Optical depth.

An irrational constant defined as the ratio of the circumference of a circle to its radius, equal to the radian measure of a full turn; approximately 6.283185307 (equal to 2π, or twice the value of π).

tauon

a voiceless dental plosive (IPA [t̪]).

A voiced coronal click consonant (dental τ̦ = IPA [ɡ͜ǀ]; cacuminal τ̣ = IPA [ɡ͜ǃ]).

Lower-case tau (ταῦ), the 19th letter of the ancient Greek alphabet. It represented the voiceless unaspirated alveolar or dental plosive /t/. It is preceded by σ and followed by υ.

The lower case letter tau (ταυ), the 19th letter of the modern Greek alphabet.

Source: wiktionary.org