improvement vs fitting

improvement and fitting both are nouns.

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

As nouns, fitting is a hyponym of improvement; that is, fitting is a word with a more specific, narrower meaning than improvement:

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

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