The Collaborative International Dictionary of English v.0.48:

Decline \De*cline"\, v. i. [imp. & p. p. Declined; p. pr. &
   vb. n. Declining.] [OE. declinen to bend down, lower, sink,
   decline (a noun), F. d['e]cliner to decline, refuse, fr. L.
   declinare to turn aside, inflect (a part of speech), avoid;
   de- + clinare to incline; akin to E. lean. See Lean, v. i.]
   1. To bend, or lean downward; to take a downward direction;
      to bend over or hang down, as from weakness, weariness,
      despondency, etc.; to condescend. "With declining head."
      --Shak.
      [1913 Webster]

            He . . . would decline even to the lowest of his
            family.                               --Lady
                                                  Hutchinson.
      [1913 Webster]

            Disdaining to decline,
            Slowly he falls, amidst triumphant cries. --Byron.
      [1913 Webster]

            The ground at length became broken and declined
            rapidly.                              --Sir W.
                                                  Scott.
      [1913 Webster]

   2. To tend or draw towards a close, decay, or extinction; to
      tend to a less perfect state; to become diminished or
      impaired; to fail; to sink; to diminish; to lessen; as,
      the day declines; virtue declines; religion declines;
      business declines.
      [1913 Webster]

            That empire must decline
            Whose chief support and sinews are of coin.
                                                  --Waller.
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            And presume to know . . .
            Who thrives, and who declines.        --Shak.
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   3. To turn or bend aside; to deviate; to stray; to withdraw;
      as, a line that declines from straightness; conduct that
      declines from sound morals.
      [1913 Webster]

            Yet do I not decline from thy testimonies. --Ps.
                                                  cxix. 157.
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   4. To turn away; to shun; to refuse; -- the opposite of
      accept or consent; as, he declined, upon principle.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

declining \declining\ adj.
   1. decreasing; as, steadily declining incomes.

   Syn: down(prenominal).
        [WordNet 1.5]

   2. going from better to worse.

   Syn: deteriorating, failing, regressing, retrograde,
        retrogressive.
        [WordNet 1.5]

   3. becoming less or smaller; as, declining powers of body and
      mind. Opposite of increasing.

   Syn: eroding.
        [WordNet 1.5]

   4. drawing to an end; waning; as, his declining years.
      [prenominal]
      [WordNet 1.5]

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "declining":
   abjuration, abjurement, abnegation, aging, chucking, chucking out,
   collapsing, coming apart, contempt, contractive, contradiction,
   cracking, crumbling, decadent, deciduous, declension, declination,
   declinature, declined, declivate, declivitous, declivous,
   decreasing, decrescendo, decrescent, decurrent, degenerate,
   deliquescent, denial, deprivation, descendant, descending,
   despisal, despising, deteriorating, diminishing, diminuendo,
   dipping, disagreement, disallowance, disapproval, discard,
   disclaimer, disclamation, discounting, disintegrating, dismissal,
   disobedience, disownment, disregard, dissent, doting, down,
   down-reaching, downcoming, downfalling, downgoing, downgrade,
   downhill, downsinking, downward, draining, drooping, dropping,
   dwindling, dying, ebbing, effete, exception, exclusion, fading,
   failing, falling, flagging, fragmenting, getting on,
   going to pieces, growing old, holding back, ignoring, languishing,
   lessening, marcescent, nay, negation, negative, negative answer,
   nix, no, nonacceptance, nonapproval, noncompliance, nonconsent,
   nonconsideration, nonobservance, on the descendant,
   on the downgrade, on the wane, passing by, pining, plummeting,
   plunging, putting away, putting out, rebuff, recantation, receding,
   reductive, refusal, regressive, rejection, renouncement,
   repudiation, repulse, retention, retiring, retreating, retrograde,
   retrogressive, sagging, scouting, senescent, setting, shrinking,
   shriveling, sinking, sliding, slipping, slumping, spurning,
   submerging, subsiding, tabetic, throwing out, thumbs-down,
   tottering, tumbledown, turndown, turning out, unwillingness,
   waning, wasting, wilting, withering, withholding, worsening