Declivity

n.	1.	Deviation from a horizontal line; gradual descent of surface; inclination downward; slope; - opposed to acclivity, or ascent; the same slope, considered as descending, being a declivity, which, considered as ascending, is an acclivity.
	2.	A descending surface; a sloping place. Commodious declivities and channels for the passage of the waters. - Derham.

Noun

1.

declivity - a downward slope or bend Synonyms: downslope, declension, declination, decline, fall, descent

declension, declination, decline, descent, downhill, downslope, fall, incline, side, slope, steep