Time constant
= Time constant
{title2=$e^{-t/\tau}$}
The time constant $\tau>0$ in <exponential decay> is the time over which the decaying quantity is multiplied by $e^{-1}$. A response approaching equilibrium as $q(t)=q_\infty+(q(0)-q_\infty)e^{-t/\tau}$ has the same time constant. For a resistor-inductor circuit it is $L/R$.