A sand bar that completely crosses a bay, sealing it off from the open ocean, is called what

A sand bar that completely crosses a bay, sealing it off from the open ocean, is called a baymouth bar.

As a result of longshore drift (which creates beaches as well), this depositional feature that doesn't allow access to a bay is created. This refusal of access is complete, not partial as is the case with a barrier island separating a lagoon. A baymouth bar consists of a lot of sand and gravel that accumulated over time.