What is a narrow piece of land connecting two larger areas across an expanse of water called?

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.

Canals are often built across isthmuses, where they may be a particularly advantageous shortcut for marine transport.

For example, the Panama Canal crosses the Isthmus of Panama, connecting the North Atlantic and Pacific Oceans; the Suez Canal connects the Mediterranean Sea and the Red Sea, cutting across the western side of the Isthmus of Suez, formed by the Sinai Peninsula.