foothill

A foothill is an area of land between mountains and the flat ground below. It's usually smaller than the mountain itself but can still be quite steep.

When we talk about hiking or climbing a mountain, you might hear someone say they're planning to start at the base of a mountain on its foothill side. This means they'll begin their journey near where the mountain starts rising up from the flat ground and work their way up from there. Some foothills are so gentle that people build houses or farms there, but others can be rugged enough for only hardy plants to survive.