The Collaborative International Dictionary of English v.0.48:

Relax \Re*lax"\ (r?-l?ks"), v. t. [imp. & p. p. Relaxed
   (-l?kst"); p. pr. & vb. n. Relaxing.] [L. relaxare; pref.
   re- re- + laxare to loose, to slacken, from laxus loose. See
   Lax, and cf. Relay, n., Release.]
   1. To make lax or loose; to make less close, firm, rigid,
      tense, or the like; to slacken; to loosen; to open; as, to
      relax a rope or cord; to relax the muscles or sinews.
      [1913 Webster]

            Horror . . . all his joints relaxed.  --Milton.
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            Nor served it to relax their serried files.
                                                  --Milton.
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   2. To make less severe or rigorous; to abate the stringency
      of; to remit in respect to strenuousness, earnestness, or
      effort; as, to relax discipline; to relax one's attention
      or endeavors.
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            The statute of mortmain was at several times relaxed
            by the legislature.                   --Swift.
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   3. Hence, to relieve from attention or effort; to ease; to
      recreate; to divert; as, amusement relaxes the mind.
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   4. To relieve from constipation; to loosen; to open; as, an
      aperient relaxes the bowels.
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   Syn: To slacken; loosen; loose; remit; abate; mitigate; ease;
        unbend; divert.
        [1913 Webster]

WordNet (r) 3.0 (2006):

relaxing
    adj 1: affording physical or mental rest; "she spent a restful
           night at home" [syn: restful, reposeful, relaxing]
           [ant: restless, uneasy]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "relaxing":
   abating, allaying, alleviating, assuaging, blunting, chastening,
   comfortable, comfy, commodious, contented, convenient, cozy,
   cushioned, cushioning, cushiony, cushy, dampening, damping,
   deadening, demulcent, diminishing, dulling, easeful, easing, easy,
   emollient, friendly, homelike, homely, homey, lessening, lived-in,
   loosening, luxurious, mitigating, mollifying, peaceful, reducing,
   reposeful, restful, roomy, snug, soft, softening, subduing,
   tempering, warm