If you're standing at the top of a hill looking down to the bottom, you're staring down a declivity, a downward slope of any kind.
The word declivity comes from the Latin words de, which means "down," and clivus, which means "slope." If you're at the bottom of the hill looking up, you'll see the opposite of a declivity. From that position you'll be looking at an acclivity, an upward slope.
