relaxation time

1 :  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 (where e is the base of natural logarithms)

2 :  the period required for the attainment of statistical equipartition of energy (as of motion) within the Milky Way galaxy, any other galaxy, groups of galaxies, clusters, stars or any selected group of similar celestial bodies

3 :  the time required for a viscous substance to recover from shearing stress after flow has ceased