1. The time taken by a disturbed system to reach equilibrium, or the time taken for the magnitude of some parameter to decrease to about 37% of its initial value. For example, the temperature-dependent relaxation time (τ‎) of a dielectric is related to the relaxation frequency (f_r) such that τ‎ = 1/(2π‎f_r). Physically, it is the time taken for an ionic defect to move within a crystal lattice under the influence of an applied alternating electromagnetic field.

2. In geomorphology, the time taken for a system to become adjusted to a sustained change in the nature and/or intensity of external processes. Such an adjustment normally involves a change in the shape of the land-form or landscape constituting the system. Relaxation times vary. The width of a river channel may adjust in response to an increase in discharge in, perhaps, 10 years, while a glaciated mountain range may require 10⁵–10⁶ years to lose the imprint of ice.