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.
      [1913 Webster]

            And presume to know . . .
            Who thrives, and who declines.        --Shak.
      [1913 Webster]

   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.
      [1913 Webster]

   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:

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.
      [1913 Webster]

            Their fathers lived in the decline of literature.
                                                  --Swift.
      [1913 Webster]

   2. (Med.) That period of a disorder or paroxysm when the
      symptoms begin to abate in violence; as, the decline of a
      fever.
      [1913 Webster]

   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.
          [1913 Webster]

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.
      [1913 Webster]

            In melancholy deep, with head declined. --Thomson.
      [1913 Webster]

            And now fair Phoebus gan decline in haste
            His weary wagon to the western vale.  --Spenser.
      [1913 Webster]

   2. To cause to decrease or diminish. [Obs.] "You have
      declined his means." --Beau. & Fl.
      [1913 Webster]

            He knoweth his error, but will not seek to decline
            it.                                   --Burton.
      [1913 Webster]

   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.
      [1913 Webster]

            Could I
            Decline this dreadful hour?           --Massinger.
      [1913 Webster]

   4. (Gram.) To inflect, or rehearse in order the changes of
      grammatical form of; as, to decline a noun or an
      adjective.
      [1913 Webster]

   Note: Now restricted to such words as have case inflections;
         but formerly it was applied both to declension and
         conjugation.
         [1913 Webster]

               After the first declining of a noun and a verb.
                                                  --Ascham.
         [1913 Webster]

   5. To run through from first to last; to repeat like a
      schoolboy declining a noun. [R.] --Shak.
      [1913 Webster]

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"

Moby Thesaurus II by Grady Ward, 1.0:

445 Moby Thesaurus words for "decline":
   abate, abatement, abjure, ablate, abstain, abstain from, age,
   analyze, anility, arrear, arrearage, arrears, ascend, asking price,
   avoid, backslide, backsliding, balk, bank, bate, be eaten away,
   be found wanting, be unmoved, be unwilling, bearish prices,
   beat down, beg off, bid price, boggle, book value, bracket, break,
   brush aside, bullish prices, bypass, cadence, caducity, call price,
   cant, careen, catabasis, catenary, cave, cave in, cessation,
   cheapen, cheapening, cheat the undertaker, childishness, chuck,
   chuck out, climb, close, closing, closing price, collapse,
   come apart, come down, come short, come unstuck, comedown,
   conjugate, conk out, consume, consume away, contemn, contradict,
   corrode, crash, crumble, cut, cut prices, debasement, debility,
   decadence, decadency, decay, deceleration, declension, declination,
   decline and fall, decline to accept, declivity, decrease,
   decrescendo, decurrence, defalcation, default, defectiveness,
   deficit, deflate, deflation, deformation, degeneracy, degenerate,
   degenerateness, degeneration, degradation, delinquency, deliquesce,
   demotion, demur, deny, depravation, depravedness, depreciate,
   depreciation, depression, derogation, descend, descent, despise,
   deteriorate, deterioration, devaluate, devaluation, devitalization,
   devolution, die, die away, diminish, diminuendo, diminution, dip,
   disagree, disallow, disapprove, discard, disclaim, discount,
   disdain, disimprove, disintegrate, dismiss, disown, disregard,
   dissent, dive, dodder, dotage, dotardism, downfall, downgate,
   downgrade, downhill, downslide, downswing, downtrend, downturn,
   downward mobility, downward trend, drift away, droop, drop,
   drop down, drop off, dwindle, dwindling, dying, ebb, ebbing,
   effeteness, erode, except, exclude, face value, fade, fading, fail,
   failing, failure, failure of nerve, faint, fall, fall away,
   fall in price, fall off, fall short, falling off, falling short,
   falling-off, falloff, fixed price, fizzle out, flag, flash price,
   flop, flop down, flump, flump down, flurry, flutter, forbear,
   forgo, forswear, founder, gainsay, get along, get on, give out,
   give way, go, go away, go down, go downhill, go off, go soft,
   go to pieces, go to pot, go uphill, grade, grow old, hang, hanging,
   have a comedown, high, hit a slump, hit rock bottom, hit the skids,
   hold out against, homestretch, hyphenate, ignore, imperfection,
   inadequacy, incline, inferiority, inflect, insufficiency,
   involution, issue par, issue price, jew down, jib, keel, lack, lag,
   languish, lapse, last lap, last round, last stage, lean, lessen,
   lessening, let up, list, lose ground, lose strength, loss,
   loss of tone, low, lower, lowering, mark, mark down, markdown,
   market price, market value, melt away, move away, move off, negate,
   negative, nominal value, nose dive, nose-dive, not answer, not buy,
   not consent, not hack it, not hear of, not make it, not make out,
   not measure up, not stretch, not suffice, not think of,
   offering price, opening price, par, par value, pare, parenthesize,
   parity, parse, pass by, pass up, peak, peg out, peter out, pine,
   pitch, plop, plop down, plummet, plummeting, plump, plunge, point,
   poop out, price, price cut, price fall, price reduction, pull away,
   punctuate, push aside, put price, quotation, quoted price, rake,
   rally, reach the depths, rebuff, recant, recede, reduce, reduction,
   refrain, refuse, refuse consent, refuse to consider, regression,
   reject, relapse, remission, renounce, repel, reprobate, repudiate,
   repulse, resist entreaty, resist persuasion, retire, retreat,
   retrocede, retrocession, retrogradation, retrograde, retrogression,
   return, revert, rise, rot, run down, run low, run short, sag,
   say nay, say no, scout, scruple, second childhood, senectitude,
   senile debility, senile dementia, senile psychosis,
   senile weakness, senilism, senility, set, settle, settle down,
   settling price, shake, shave, shelve, short measure, shortage,
   shortcoming, shortfall, shove away, shrink, shrivel, shy, sidle,
   sink, sink down, sinkage, slant, slash, slide, slip, slippage,
   slope, slouch, slowdown, slump, slump down, spurn, stand aloof,
   stand off, stated value, stick, stickle, stop short, submerge,
   submergence, subside, subsidence, swag, sway, swings, tail off,
   taper off, throw away, throw out, tilt, tip, totter, touch bottom,
   trim, turn away, turn down, turn gray, turn out, turn white,
   underage, uprise, veto, vote nay, vote negatively, waive, wane,
   waning, want, waste, waste away, weaken, weakening, weakness, wear,
   wear away, wear thin, widen the distance, wilt, withdraw, wither,
   wither away, wizen, worsen, worsening, wrinkle, yield