What do you call a spit that joins an island to the mainland?

The correct term for a spit that connects an island to the mainland is a tombolo. A tombolo typically forms when sediment is deposited by waves or tidal actions, creating a narrow landform that links the island to the nearest land. This phenomenon occurs due to the way waves behave around islands, causing sand and other materials to accumulate in certain areas.

In contrast, a spit refers to a stretch of beach material that protrudes into the sea and is not necessarily connected to an island. A lagoon is a shallow body of water separated from a larger sea or ocean by landforms such as barriers or spits, and a barrier island is a coastal landform that runs parallel to the mainland but remains distinct from it. Thus, the unique process and characteristics of a tombolo make it the proper term for a spit that connects an island to the mainland.