decline vs reject

decline and reject both are nouns.

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

As verbs, reject is a hyponym of decline; that is, reject is a word with a more specific, narrower meaning than decline:

Other hyponyms of decline include disdain, freeze off, pooh-pooh, scorn, spurn, turn down, contract out, regret, repudiate, disobey.

As verbs, decline and reject are synonyms defined as:

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

decline (noun)	reject (noun)
a downward slope or bend	the person or thing that is rejected or set aside as inferior in quality
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

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