A capital Greek letter tau commonly used to represent torque in physics or time constants in engineering.
Examples
Time period in physics
\Tau = \frac{1}{f} \text{ where f is frequency}
Torque vector in mechanics
\vec{\Tau} = \vec{r} \times \vec{F}
Mean lifetime in particle physics
\Tau_{\mu} = 2.197 \times 10^{-6} \text{ s}