steep vs declivity

steep and declivity both are nouns.

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

As nouns, declivity is a hypernym of steep; that is, declivity is a word with a broader meaning than steep:

Other hypernyms of steep include declension, declination, decline, descent, downslope, fall.

steep (noun)	declivity (noun)
a steep place (as on a hill)	a downward slope or bend

steep (adjective)	declivity (adjective)
having a sharp inclination
of a slope; set at a high angle
greatly exceeding bounds of reason or moderation

steep (verb)	declivity (verb)
let sit in a liquid to extract a flavor or to cleanse
devote (oneself) fully to