de·cliv·i·ty /dɪˈklɪvɪti/

–noun, plural -ties.
a downward slope, as of ground ( opposed to acclivity).

Origin:
1605–15; < L of dēclīvitās a slope, hill, equiv. to dēclīvi ( s ) sloping downward ( dē– de- + clīv ( us ) slope, hill + –is adj. suffix) + –tās -ty

Source: Dictionary.com.

The declivity was so small, that I walked near a mile before I got to the shore

Source: Gulliver’s Travels by Jonathan Swift.