A foothill.
A low hill at the base of a mountain is typically called a foothill. It serves as a transition between the flatlands and the more rugged terrain of the mountain. Foothills are often characterized by gentle slopes and can be rich in vegetation.
The low path at the bottom of the hill is commonly referred to as a valley or a ravine.
Its a hill
Valley
valleys
A valley
A synonym for mountain could possibly be a hill.
How high it is. Too low, and it's considered a hill instead.
A low rolling hill can also be called a knoll or a mound.
The low point of a mountain range is called a pass. In contrast, the high point is called a peak.
A low point on a ridge between two mountain peaks is called a saddle. A low level separating two mountains is called a valley.
There is no universally agreed definition of mountain; in the UK anything over 600m is considered a mountain, but in the USA some landscapes as low as 30m are called "mountains", also in the Gambia there is a "mountain" of 53m height. The short answer, with such an arbitary difference between hill and mountain, its impossible to say.