For highways, roads, trails, railroads, and so forth that are designed (as opposed to just trails that came into existence by people using animal trails, old mule trails, and such) the inclines and descents are designed using percent. It originates from railway engineering (or even older, from the Romans with their aquaducts).
Grade is expressed in percent (amount of vertical change in some horizontal distance, that ratio being multiplied by 100 to become a percent).
A two foot rise in 100 feet is a +2.00 percent grade ((2/100)*100).
A two foot decline in 100 feet is a -2.00 percent grade.
A 100 percent grade is a 45 degree angle (100 feet up [or down] in 100 feet horizontal).
A very steep slope (running or vehicles) is 12-15 percent.
Whenever people decide to use both grades, slopes, gradients, and angles the confusion starts. The mind almost always thinks of a 100 percent grade as vertical.