WordNet sense 1 ((physics) the exponential return of a system to equilibrium after a disturbance):

relaxation behavior

WordNet sense 2 (a feeling of refreshing tranquility and an absence of tension or worry):

easiness

WordNet sense 3 (an occurrence of control or strength weakening):

loosening, slackening

WordNet sense 4 (freedom from activity (work or strain or responsibility)):

ease, repose, rest

WordNet sense 5 (a method of solving simultaneous equations by guessing a solution and then reducing the errors that result by successive approximations until all the errors are less than some specified amount):

relaxation method

WordNet sense 6 (the act of making less strict):

liberalization, liberalisation