The Collaborative International Dictionary of English v.0.48:

Improvement \Im*prove"ment\, n.
   1. The act of improving; advancement or growth; promotion in
      desirable qualities; progress toward what is better;
      melioration; as, the improvement of the mind, of land,
      roads, etc.
      [1913 Webster]

            I look upon your city as the best place of
            improvement.                          --South.
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            Exercise is the chief source of improvement in all
            our faculties.                        --Blair.
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   2. The act of making profitable use or applicaton of
      anything, or the state of being profitably employed; a
      turning to good account; practical application, as of a
      doctrine, principle, or theory, stated in a discourse. "A
      good improvement of his reason." --S. Clarke.
      [1913 Webster]

            I shall make some improvement of this doctrine.
                                                  --Tillotson.
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   3. The state of being improved; betterment; advance; also,
      that which is improved; as, the new edition is an
      improvement on the old.
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            The parts of Sinon, Camilla, and some few others,
            are improvements on the Greek poet.   --Addison.
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   4. Increase; growth; progress; advance.
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            There is a design of publishing the history of
            architecture, with its several improvements and
            decays.                               --Addison.
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            Those vices which more particularly receive
            improvement by prosperity.            --South.
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   5. pl. Valuable additions or betterments, as buildings,
      clearings, drains, fences, etc., on land or premises; as,
      to convey the land together with all improvements.
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   6. (Patent Laws) A useful addition to, or modification of, a
      machine, manufacture, or composition. --Kent.
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WordNet (r) 3.0 (2006):

improvement
    n 1: a change for the better; progress in development [syn:
         improvement, betterment, advance]
    2: the act of improving something; "their improvements increased
       the value of the property"
    3: a condition superior to an earlier condition; "the new school
       represents a great improvement" [syn: improvement,
       melioration] [ant: declination, decline]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "improvement":
   about-face, accommodation, adaptation, adjustment, advance,
   alteration, amelioration, amendment, apostasy, apprenticeship,
   basic training, betterment, break, breaking, breeding, change,
   change of allegiance, change of heart, change of mind,
   changeableness, conditioning, constructive change, continuity,
   convalescence, conversion, cultivation, defection, degeneration,
   degenerative change, deterioration, development, deviation,
   difference, discipline, discontinuity, divergence, diversification,
   diversion, diversity, drill, drilling, enhancement, exercise,
   fetching-up, fitting, flip-flop, fostering, gain, gradual change,
   grooming, housebreaking, in-service training, increase,
   instauration, manual training, melioration, military training,
   mitigation, modification, modulation, new birth, nurture,
   nurturing, on-the-job training, overthrow, practice, preparation,
   progress, qualification, radical change, raising, re-creation,
   reactivation, readying, realignment, rearing, rebirth, reclamation,
   reconstitution, reconversion, recovery, recrudescence,
   recuperation, redemption, redesign, redintegration, reenactment,
   reestablishment, reform, reformation, regeneration, rehabilitation,
   rehearsal, reinstatement, reinstation, reinstitution,
   reinvestiture, reinvestment, remaking, renascence, renewal, repair,
   replacement, reshaping, restitution, restoration, restructuring,
   reversal, reversion, revival, revivification, revolution, rise,
   shift, sloyd, sudden change, switch, total change, training,
   transition, turn, turnabout, upbringing, upgrading, upheaval,
   upswing, variation, variety, violent change, vocational education,
   vocational training, worsening



Bouvier's Law Dictionary, Revised 6th Ed (1856):

IMPROVEMENT, rights. An addition of some useful thing to a machine,
manufacture or composition of matter.
     2. The patent law of July 4, 1836, authorizes the granting of a patent
for any new and useful improvement on any art, machine manufacture or
composition of matter. Sect. 6. It is often very difficult to say what is a
new and useful improvement, the cases often approach very near to each
other. In the present improved state of machinery, it is almost
impracticable not to employ the same elements of motion, and in some
particulars, the same manner of operation, to produce any new effect. 1
Gallis. 478; 2 Gallis. 51. See 4 B. & Ald. 540; 2 Kent, Com. 370.



Bouvier's Law Dictionary, Revised 6th Ed (1856):

IMPROVEMENT, estates. This term is of doubtful meaning It would seem to
apply principally to buildings, though generally it extends to amelioration
of every description of property, whether real or personal; it is generally
explained by other words.
     2. Where, by the terms of a lease, the covenant was to leave at the end
of the term a water-mill with all the fixtures, fastenings, and
improvements, during the demise fixed, fastened, or set up on or upon the
premises, in good plight and condition, it was held to include a pair of new
millstones set up by the lessee during the term, although the custom of the
country in general authorized the tenant to remove them. 9 Bing. 24; 3 Sim.
450; 2 Ves. & Bea. 349. Vide 3 Yeates, 71; Addis. R. 335; 4 Binn. R. 418; 5
Binn. R. 77; 5 S. & R. 266; 1 Binn. R. 495; 1 John. Ch. R. 450; 15 Pick. R.
471. Vide Profits. 2 Man. & Gra. 729, 757; S. C. 40 Eng. C. L. R. 598, 612.
     3. Tenants in common are not bound to pay for permanent improvements,
made on the common property, by one of the tenants in common without their
consent. 2 Bouv. Inst. n. 1881.