fitting vs improvement

fitting and improvement both are nouns.

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

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

Other hypernyms of fitting include advance, betterment.

fitting (noun)	improvement (noun)
putting clothes on to see whether they fit	the act of improving something
a small and often standardized accessory to a larger system	a change for the better; progress in development
(usually in the plural) furnishings and equipment (especially for a ship or hotel)	a condition superior to an earlier condition
making or becoming suitable; adjusting to circumstances

fitting (adjective)	improvement (adjective)
being precisely fitting and right
in harmony with the spirit of particular persons or occasion