improver vs additive

improver and additive both are nouns.

Word	Noun	Adjective	Verb	Adverb
improver	Yes	No	No	No
additive	Yes	Yes	No	No

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

Other hyponyms of improver include afterthought, annex, annexe, extension, wing, attachment, elongation, extension.

improver (noun)	additive (noun)
a component that is added to something to improve it	something added to enhance food or gasoline or paint or medicine
someone devoted to the promotion of human welfare and to social reforms

improver (adjective)	additive (adjective)
	characterized or produced by addition
	designating or involving an equation whose terms are of the first degree