melioration

Noun

• the linguistic process in which over a period of time a word grows more positive in connotation or more elevated in meaning

• a condition superior to an earlier condition
  • the new school represents a great improvement
    

• the act of relieving ills and changing for the better

Related words

• Synonyms: 
  improvement , amelioration , betterment
• Antonyms: 
  decline , declination