The Collaborative International Dictionary of English v.0.48:

Meliorism \Mel"io*rism\, n. [From L. melior better.]
   The doctrine that there is a tendency throughout nature
   toward improvement. --J. Sully.
   [1913 Webster]

WordNet (r) 3.0 (2006):

meliorism
    n 1: the belief that the world can be made better by human
         effort