Wiktionary
n. An island close to, and parallel to, the main coastline.
WordNet
n. a long narrow sandy island (wider than a reef) running parallel to the shore
Wikipedia
Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from a few islands to more than a dozen. They are subject to change during storms and other action, but absorb energy and protect the coastlines and create areas of protected waters where wetlands may flourish. A barrier chain may extend uninterrupted for over a hundred kilometers, excepting the tidal inlets that separate the islands, the longest and widest being Padre Island of Texas. The length and width of barriers and overall morphology of barrier coasts are related to parameters including tidal range, wave energy, sediment supply, sea-level trends, and basement controls.
Chains of barrier islands can be found along approximately thirteen percent of the world's coastlines. They display different settings, suggesting that they can form and be maintained in a variety of environmental settings. Numerous theories have been given to explain their formation.
Usage examples of "barrier island".
Damian clung to the edge of the hatchway, peered out the tiny window toward the Old Dike and the cliffs that marked the edge of Barrier Island.
I became increasingly morose over the next two or three days, because it became clear to us as we traipsed up and down endless hills in the rain, that we were not going to find a kakapo on Little Barrier Island.
We had landed on a narrow barrier island, but the mainland was but a short distance away.
Brinkman's task was to make a list of species that lived on the small barrier island: plants, insects, birds, amphibians, reptiles and mammals.