From The Collaborative International Dictionary of English v.0.48 :
Melioration \Mel`io*ra"tion\, n. [L. melioratio.]
The act or operation of meliorating, or the state of being
meliorated; improvement. --Bacon.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
melioration
n 1: the act of relieving ills and changing for the better [syn:
amelioration, melioration, betterment]
2: a condition superior to an earlier condition; "the new school
represents a great improvement" [syn: improvement,
melioration] [ant: declination, decline]
3: the linguistic process in which over a period of time a word
grows more positive in connotation or more elevated in
meaning