Question

define the terms drift velocity and relaxation time

Answer 1

The average velocity attained by some particle such as an electron due to the influence of an electric field is termed as the drift velocity.
the time required for an exponentially decreasing variable (as the amplitude of a damped oscillation) to drop from an initial value to 1/e or 0.368 of that value 

Answer 2

Drift velocity: it is a average velocity that a particle, such as an electron , attains in a material due to an electric field

Relaxation time: The time required for an exponentially decreasing variable .....initial value to 1/e or 0.368 of that value!