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.