A foothill is one of the smaller hills that sit below a group of high mountains, at the place where the land starts to rise. You nearly always hear the plural: the foothills. Travellers, geography books, and weather reports use it. The “foot” here is the bottom or base, as in the foot of the mountain — not the body part doing the walking, though the picture is related. A foothill is not the high peak, and it is not the flat plain in front; it is the lower, kinder ground in between. People sometimes stretch it in writing (“the foothills of a career”) to mean the early, lower slopes of something big, but the home meaning is the hills themselves.