Isthmus

Narrow strip of land connecting two larger land areas

An isthmus is a narrow piece of land connecting two larger areas across an expanse of water by which they are otherwise separated. A tombolo is an isthmus that consists of a spit or bar, and a strait is the sea counterpart of an isthmus, a narrow stretch of sea between two landmasses that connects two larger bodies of water.

This keyword could refer to multiple things. Here are some suggestions: