declivity
Ground slanting down
A declivity is a downward slope or incline.
The word is somewhat formal or literary, appearing more often in geographic or architectural descriptions than casual speech: The path followed the declivity toward the river.
The opposite is acclivity, an upward slope.
slope
Rate of change across space
gradient
Rate of change across space
pitch · slant · tilt · angle · steepness · fall · drop · dip · cant · hillside
ravine
Deep, narrow valley
hollow · valley · gorge · canyon · defile · basin · couloir · draw · wash · arroyo
precipitous
Extremely steep, almost vertical
vertical · plunging · sharp · severe · dramatic · gentle · gradual · moderate · mild · slight