melioration vs improvement

melioration and improvement both are nouns.

Word	Noun	Adjective	Verb	Adverb
melioration	Yes	No	No	No
improvement	Yes	No	No	No

As nouns, improvement is a hypernym of melioration; that is, improvement is a word with a broader meaning than melioration:

As nouns, melioration and improvement are synonyms defined as:

melioration and improvement: a condition superior to an earlier condition

melioration (noun)	improvement (noun)
the act of relieving ills and changing for the better	the act of improving something
the linguistic process in which over a period of time a word grows more positive in connotation or more elevated in meaning	a change for the better; progress in development
a condition superior to an earlier condition	a condition superior to an earlier condition