The Collaborative International Dictionary of English v.0.48:

Improve \Im*prove"\, v. t. [imp. & p. p. Improved; p. pr. &
   vb. n. Improving.] [Pref. in- in + prove, in approve. See
   Approve, Prove.]
   1. To make better; to increase the value or good qualities
      of; to ameliorate by care or cultivation; as, to improve
      land. --Donne.
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            I love not to improve the honor of the living by
            impairing that of the dead.           --Denham.
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   2. To use or employ to good purpose; to make productive; to
      turn to profitable account; to utilize; as, to improve
      one's time; to improve his means. --Shak.
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            We shall especially honor God by improving
            diligently the talents which God hath committed to
            us.                                   --Barrow.
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            A hint that I do not remember to have seen opened
            and improved.                         --Addison.
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            The court seldom fails to improve the opportunity.
                                                  --Blackstone.
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            How doth the little busy bee
            Improve each shining hour.            --I. Watts.
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            Those moments were diligently improved. --Gibbon.
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            True policy, as well as good faith, in my opinion,
            binds us to improve the occasion.     --Washington.
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   3. To advance or increase by use; to augment or add to; --
      said with reference to what is bad. [R.]
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            We all have, I fear, . . . not a little improved the
            wretched inheritance of our ancestors. --Bp.
                                                  Porteus.

   Syn: To better; meliorate; ameliorate; advance; heighten;
        mend; correct; rectify; amend; reform.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Improve \Im*prove"\, v. t. [Pref. im- not + prove: cf. L.
   improbare, F. improuver.]
   1. To disprove or make void; to refute. [Obs.]
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            Neither can any of them make so strong a reason
            which another can not improve.        --Tyndale.
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   2. To disapprove; to find fault with; to reprove; to censure;
      as, to improve negligence. [Obs.] --Chapman.
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            When he rehearsed his preachings and his doing unto
            the high apostles, they could improve nothing.
                                                  --Tyndale.
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The Collaborative International Dictionary of English v.0.48:

Improve \Im*prove"\, v. i.
   1. To grow better; to advance or make progress in what is
      desirable; to make or show improvement; as, to improve in
      health.
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            We take care to improve in our frugality and
            diligence.                            --Atterbury.
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   2. To advance or progress in bad qualities; to grow worse.
      "Domitian improved in cruelty." --Milner.
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   3. To increase; to be enhanced; to rise in value; as, the
      price of cotton improves.
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   To improve on or To improve upon, to make useful
      additions or amendments to, or changes in; to bring nearer
      to perfection; as, to improve on the mode of tillage.
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WordNet (r) 3.0 (2006):

improve
    v 1: to make better; "The editor improved the manuscript with
         his changes" [syn: better, improve, amend,
         ameliorate, meliorate] [ant: aggravate, exacerbate,
         exasperate, worsen]
    2: get better; "The weather improved toward evening" [syn:
       better, improve, ameliorate, meliorate] [ant:
       decline, worsen]

Moby Thesaurus II by Grady Ward, 1.0:

235 Moby Thesaurus words for "improve":
   accommodate, acculturate, adapt, adjust, advance, alter,
   ameliorate, amend, apprentice, be changed, be converted into,
   be renewed, benefit, benefit from, best, better, boost, bottom out,
   brace up, break, break in, break up, breed, bring forward,
   bring up, cap, capitalize on, cash in on, change, checker, chop,
   chop and change, civilize, come about, come along, come around,
   come on, come round, complete, condition, convalesce, convert,
   correct, crown, culminate, cultivate, deform, degenerate, denature,
   deteriorate, develop, deviate, discipline, diverge, diversify,
   do to perfection, drill, edify, edit, educate, elevate, emend,
   enhance, enlighten, enrich, exceed, excel, exercise, exploit,
   fatten, favor, fetch up, fit, fix up, flop, form, forward, foster,
   further, gain, gain ground, gain strength, get ahead, get along,
   get better, go ahead, go forward, go one better, go straight,
   graduate, groom, grow better, haul around, help, house-train,
   housebreak, illume, illumine, improve on, improve the occasion,
   improve upon, increase, irradiate, jibe, lard, lick into shape,
   lift, look up, make an improvement, make capital of, make hay,
   make headway, make progress, make strides, mature, meliorate, mend,
   mitigate, modernize, modify, modulate, mutate, nurse, nurture,
   outweigh, overbalance, overbear, overcome, overhaul, overpass,
   overthrow, overtop, perfect, perk up, pick up, practice,
   predominate, prepare, preponderate, prevail, profit by, progress,
   promote, put in tune, put right, put to advantage, put to school,
   qualify, raise, rally, re-create, ready, realign, rear, rebuild,
   recondition, reconstruct, recover, recruit, rectify, recuperate,
   redeem, redesign, redress, refine, refine upon, refit, reform,
   refurbish, rehabilitate, rehearse, remake, remedy, remodel, renew,
   renovate, repair, reshape, restructure, revamp, revise, revive,
   ring the changes, ripen, rub up, send to school, set right,
   shape up, shift, shift the scene, show improvement,
   shuffle the cards, skyrocket, sleep it off, socialize,
   straighten out, strengthen, subvert, surpass, swerve, tack,
   take a turn, take advantage of, take in hand, take off, top,
   tower above, tower over, trade on, train, transcend, transfigure,
   transform, trump, turn, turn aside, turn into, turn the corner,
   turn the scale, turn the tables, turn the tide, turn to account,
   turn to profit, turn to use, turn upside down, undergo a change,
   update, upgrade, uplift, use to advantage, vary, veer, warp,
   work a change, worsen