What is relaxation time $(\tau )$? And what is drift velocity $(v_d)$?

(N/A) $1$. Relaxation Time $(\tau )$: It is defined as the average time interval between two successive collisions of an electron with the positive ions in a conductor. It is typically of the order of $10^{-14} \ s$.
$2$. Drift Velocity $(v_d)$: It is defined as the average velocity acquired by the free electrons in a conductor in the opposite direction of the externally applied electric field. It is given by the formula $v_d = -\frac{eE\tau}{m}$,where $e$ is the charge of an electron,$E$ is the electric field,$\tau$ is the relaxation time,and $m$ is the mass of the electron.