incline vs declivity

incline and declivity both are nouns.

Word	Noun	Adjective	Verb	Adverb
incline	Yes	No	Yes	No
declivity	Yes	No	No	No

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

incline (noun)	declivity (noun)
an inclined surface connecting two levels	a downward slope or bend
an elevated geological formation

incline (verb)	declivity (verb)
make receptive or willing towards an action or attitude or belief
feel favorably disposed or willing
be at an angle
lower or bend (the head or upper body), as in a nod or bow
bend or turn (one's ear) towards a speaker in order to listen well
have a tendency or disposition to do or be something; be inclined