a narrow or low passage through a mountain ridge or descending from a high elevation to a valley: Cumberland Gap, Ketner Gap.
Town: a town located at a gap between hills, providing a good defensive site and route centre that led to a trade and market function.
a pass between mountain peaks
This is a low point along a line of hills or mountains through which roads and railways can pass through.
A low point or opening between hills or mountains or in a ridge or mountain range.