The Collaborative International Dictionary of English v.0.48:

Relaxation \Re`lax*a"tion\ (r?`l?ks-?"sh?n;277), n. [L.
   relaxatio; cf. F. relaxation.]
   1. The act or process of relaxing, or the state of being
      relaxed; as, relaxation of the muscles; relaxation of a
      law.
      [1913 Webster]

   2. Remission from attention and effort; indulgence in
      recreation, diversion, or amusement. "Hours of careless
      relaxation." --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

relaxation
    n 1: (physiology) the gradual lengthening of inactive muscle or
         muscle fibers
    2: (physics) the exponential return of a system to equilibrium
       after a disturbance [syn: relaxation, relaxation
       behavior]
    3: a feeling of refreshing tranquility and an absence of tension
       or worry; "the easiness we feel when sleeping" [syn:
       easiness, relaxation]
    4: an occurrence of control or strength weakening; "the
       relaxation of requirements"; "the loosening of his grip";
       "the slackening of the wind" [syn: relaxation, loosening,
       slackening]
    5: freedom from activity (work or strain or responsibility);
       "took his repose by the swimming pool" [syn: rest, ease,
       repose, relaxation]
    6: 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 [syn: relaxation, relaxation
       method]
    7: the act of making less strict [syn: liberalization,
       liberalisation, relaxation]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "relaxation":
   abatement, abridgment, allayment, alleviation, amusement,
   assuagement, attenuation, attrition, bagginess, blunting, calming,
   carelessness, comfort, contraction, cushioning, dampening, damping,
   deadening, debilitation, decrease, decrement, decrescence,
   deduction, deflation, demulsion, depreciation, depression,
   devitalization, dilution, diminishment, diminution, diversion,
   divertisement, divertissement, dulcification, dulling, dying,
   dying off, ease, easiness, easing, easygoingness, effemination,
   enervation, enfeeblement, enjoyment, entertainment, evisceration,
   exhaustion, extenuation, fade-out, falling-off, fatigue, fun,
   hushing, impotence, imprecision, inactivity, inanition,
   indifference, languishment, laxation, laxity, laxness, leisure,
   leniency, lessening, letdown, letup, lightening, looseness,
   loosening, lowering, lulling, mellowing, miniaturization, mirth,
   mitigation, moderation, modulation, mollification, mollifying,
   negligence, overindulgence, overpermissiveness, pacification,
   padding, palliation, passe-temps, pastime, permissiveness,
   pleasure, quiet, quietening, quieting, recreation, reduction,
   regalement, relaxedness, relief, remission, remissness, repose,
   rest, restfulness, ricketiness, sagging, scaling down, shakiness,
   simplicity, slackening, slackness, sleep, sloppiness, softening,
   softening-up, softness, solace, soothing, subduement, subtraction,
   tempering, thinning, tranquilization, tranquillity,
   unbuttoned ease, unrestraint, weakening, weakness