relaxation time

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The time required for an exponential system to return 1/e (0.368) of its initial value of equilibrium after perturbation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The characteristic time for a system to reach an equilibrium condition after a disturbance
• noun medicine The time taken for a muscle to revert to its resting state after being used

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the time constant of an exponential return of a system to equilibrium after a disturbance