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]