slope vs steepness

slope and steepness both are nouns.

Word	Noun	Adjective	Verb	Adverb
slope	Yes	No	Yes	No
steepness	Yes	No	No	No

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

slope: the property possessed by a line or surface that departs from the horizontal
steepness: the property possessed by a slope that is very steep

Other hyponyms of slope include grade, pitch, rake, slant, abruptness, precipitousness, gentleness, gradualness.

slope (noun)	steepness (noun)
the property possessed by a line or surface that departs from the horizontal	the property possessed by a slope that is very steep
an elevated geological formation

slope (verb)	steepness (verb)
be at an angle