An isthmus is a thin strip of land that connects two larger land masses. It has water on both of its sides and is sometimes a tombolo, which is formed from deposits such as sand, gravel, or silt. A tombolo usually connects an island to the mainland.
There are many isthmus examples throughout the world. Although some are made of land and others of more granular particles such as sand, what they all have in common is water on both sides.