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.
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            He . . . would decline even to the lowest of his
            family.                               --Lady
                                                  Hutchinson.
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            Disdaining to decline,
            Slowly he falls, amidst triumphant cries. --Byron.
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            The ground at length became broken and declined
            rapidly.                              --Sir W.
                                                  Scott.
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   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.
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            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.
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            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.
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The Collaborative International Dictionary of English v.0.48:

Decline \De*cline"\, n. [F. d['e]clin. See Decline, v. i.]
   1. A falling off; a tendency to a worse state; diminution or
      decay; deterioration; also, the period when a thing is
      tending toward extinction or a less perfect state; as, the
      decline of life; the decline of strength; the decline of
      virtue and religion.
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            Their fathers lived in the decline of literature.
                                                  --Swift.
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   2. (Med.) That period of a disorder or paroxysm when the
      symptoms begin to abate in violence; as, the decline of a
      fever.
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   3. A gradual sinking and wasting away of the physical
      faculties; any wasting disease, esp. pulmonary
      consumption; as, to die of a decline. --Dunglison.

   Syn: Decline, Decay, Consumption.

   Usage: Decline marks the first stage in a downward progress;
          decay indicates the second stage, and denotes a
          tendency to ultimate destruction; consumption marks a
          steady decay from an internal exhaustion of strength.
          The health may experience a decline from various
          causes at any period of life; it is naturally subject
          to decay with the advance of old age; consumption may
          take place at almost any period of life, from disease
          which wears out the constitution. In popular language
          decline is often used as synonymous with consumption.
          By a gradual decline, states and communities lose
          their strength and vigor; by progressive decay, they
          are stripped of their honor, stability, and greatness;
          by a consumption of their resources and vital energy,
          they are led rapidly on to a completion of their
          existence.
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The Collaborative International Dictionary of English v.0.48:

Decline \De*cline"\, v. t.
   1. To bend downward; to bring down; to depress; to cause to
      bend, or fall.
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            In melancholy deep, with head declined. --Thomson.
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            And now fair Phoebus gan decline in haste
            His weary wagon to the western vale.  --Spenser.
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   2. To cause to decrease or diminish. [Obs.] "You have
      declined his means." --Beau. & Fl.
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            He knoweth his error, but will not seek to decline
            it.                                   --Burton.
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   3. To put or turn aside; to turn off or away from; to refuse
      to undertake or comply with; reject; to shun; to avoid;
      as, to decline an offer; to decline a contest; he declined
      any participation with them.
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            Could I
            Decline this dreadful hour?           --Massinger.
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   4. (Gram.) To inflect, or rehearse in order the changes of
      grammatical form of; as, to decline a noun or an
      adjective.
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   Note: Now restricted to such words as have case inflections;
         but formerly it was applied both to declension and
         conjugation.
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               After the first declining of a noun and a verb.
                                                  --Ascham.
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   5. To run through from first to last; to repeat like a
      schoolboy declining a noun. [R.] --Shak.
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WordNet (r) 3.0 (2006):

decline
    n 1: change toward something smaller or lower [syn: decline,
         diminution]
    2: a condition inferior to an earlier condition; a gradual
       falling off from a better state [syn: decline,
       declination] [ant: improvement, melioration]
    3: a gradual decrease; as of stored charge or current [syn:
       decay, decline]
    4: a downward slope or bend [syn: descent, declivity,
       fall, decline, declination, declension, downslope]
       [ant: acclivity, ascent, climb, raise, rise,
       upgrade]
    v 1: grow worse; "Conditions in the slum worsened" [syn:
         worsen, decline] [ant: ameliorate, better,
         improve, meliorate]
    2: refuse to accept; "He refused my offer of hospitality" [syn:
       refuse, reject, pass up, turn down, decline] [ant:
       accept, have, take]
    3: show unwillingness towards; "he declined to join the group on
       a hike" [syn: refuse, decline] [ant: accept, consent,
       go for]
    4: grow smaller; "Interest in the project waned" [syn:
       decline, go down, wane]
    5: go down; "The roof declines here"
    6: go down in value; "the stock market corrected"; "prices
       slumped" [syn: decline, slump, correct]
    7: inflect for number, gender, case, etc., "in many languages,
       speakers decline nouns, pronouns, and adjectives"