reject vs decline

reject and decline both are nouns.

Word	Noun	Adjective	Verb	Adverb
reject	Yes	No	Yes	No
decline	Yes	No	Yes	No

As verbs, decline is a hypernym of reject; that is, decline is a word with a broader meaning than reject:

Other hypernyms of reject include refuse.

As verbs, reject and decline are synonyms defined as:

Other synonyms of reject include pass up, refuse, turn down.

reject (noun)	decline (noun)
the person or thing that is rejected or set aside as inferior in quality	a downward slope or bend
	a gradual decrease; as of stored charge or current
	change toward something smaller or lower
	a condition inferior to an earlier condition; a gradual falling off from a better state; decline

reject (verb)	decline (verb)
not accept something given or offered	grow smaller
dismiss from consideration or a contest	grow worse
deem wrong or inappropriate	show unwillingness towards
reject with contempt	inflect for number, gender, case, etc.
not accept as true	go down
refuse entrance or membership	fall in value
resist immunologically the introduction of some foreign tissue or organ	not accept as true