A foothill is a low hill at the base of a mountain or mountain range. Picture standing in a flat valley and looking toward a towering mountain peak: those smaller, rounded hills between you and the mountain are foothills. They're like the mountain's front yard, gradually rising from flat ground toward the dramatic heights above.

Foothills form when erosion wears away rock and soil from mountains over millions of years, creating these gentler slopes. The foothills of the Rocky Mountains stretch across Colorado and Wyoming, while California's Central Valley meets the Sierra Nevada through rolling foothills dotted with oak trees.

The word captures something in-between: foothills aren't quite mountains, but they're definitely not flat plains either. Towns built in the foothills enjoy mountain views without the harsh weather of higher elevations. Hikers often warm up in the foothills before tackling steeper mountain trails. If someone says they live “in the foothills,” they mean they're nestled at the base of mountains, where the land starts its climb toward the sky.