A barrier island is a coastal landform that runs parallel to the mainland, typically separated by a lagoon or estuary. These islands are formed by the accumulation of sand and sediment, often shaped by ocean currents and wave action. They serve as a protective barrier for the mainland against storms and high waves. Barrier islands are important ecosystems that provide habitats for various wildlife, including birds and marine species. They also play a crucial role in coastal protection and can help reduce erosion. Popular examples of barrier islands include the Outer Banks in North Carolina and Fire Island in New York.