Amelioration is when a word improves in meaning over time due to a semantic shift..It is the opposite of Perjoration..!!
Here is my list of perjoration words..(hope I did this right)..
Bag ..(Originally meaning disparaging slang for a woman)
Rap ..(Originally meaning a quick, light tap)
Hunk ..(Originally meaning a large piece cut off)
Can ..(Originally meaning to fire an employee)