From MarineSpecies Traits Wiki
Definition of Barrier island:
A barrier islands is a ridge of sand that runs parallel to the main coast but is separated from it by a bay or lagoon.
- A detached portion of a barrier beach between two inlets.
- An offshore sand bar that rises above the mean high-water level, forming an island.
- An sandy island formed on a barrier reef. .
This is the common definition for Barrier island, other definitions can be discussed in the article
Classification of sandy coastlines