The Collaborative International Dictionary of English v.0.48:

Betterment \Bet"ter*ment\, n.
   1. A making better; amendment; improvement. --W. Montagu.
      [1913 Webster]

   2. (Law) An improvement of an estate which renders it better
      than mere repairing would do; -- generally used in the
      plural. [U. S.] --Bouvier.
      [1913 Webster]

WordNet (r) 3.0 (2006):

betterment
    n 1: a change for the better; progress in development [syn:
         improvement, betterment, advance]
    2: an improvement that adds to the value of a property or
       facility
    3: the act of relieving ills and changing for the better [syn:
       amelioration, melioration, betterment]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "betterment":
   Great Leap Forward, about-face, accommodation, adaptation,
   adjustment, advance, advancement, alteration, amelioration,
   amendment, apostasy, ascent, bettering, boost, break, change,
   change of heart, changeableness, constructive change, continuity,
   conversion, defection, degeneration, degenerative change,
   deterioration, deviation, difference, discontinuity, divergence,
   diversification, diversion, diversity, enhancement, enrichment,
   eugenics, euthenics, fitting, flip-flop, furtherance,
   gradual change, headway, improvement, lift, melioration, mend,
   mending, mitigation, modification, modulation, overthrow, pickup,
   preferment, progress, progression, promotion, qualification,
   radical change, re-creation, realignment, recovery, redesign,
   reform, reformation, remaking, renewal, reshaping, restoration,
   restructuring, reversal, revival, revivification, revolution, rise,
   shift, sudden change, switch, total change, transition, turn,
   turnabout, upbeat, upheaval, uplift, upping, upswing, uptrend,
   upward mobility, variation, variety, violent change, worsening