The word acclivity traces back to the Latin word acclivis, meaning “ascending,” which is a combination of ad-, meaning “toward,” and clivus, meaning “slope.” If you encounter an acclivity, it’s going to be all uphill until you get to the top. The opposite of an acclivity is a declivity, which has a similar Latin origin. In the case of declivity, it’s the de- prefix, meaning “down,” that moves things in the opposite direction and gives it the meaning of “downhill slope.”