The Collaborative International Dictionary of English v.0.48:
Declivity \De*cliv"i*ty\, n.; pl. Declivities. [L. declivitas,
fr. declivis sloping, downhill; de + clivus a slope, a hill;
akin to clinare to incline: cf. F. d['e]clivit['e]. See
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.