Final answer:
A barrier island fits the description of an offshore sand bar that forms above the high water level and is created by overwash from waves. These islands are part of the nearshore zone and protect the coastline, including bays and lagoons.
Step-by-step explanation:
A barrier island is a type of offshore sand bar that forms due to abundant sediment supply and sits above the mean high water level. These landforms are typically created by the process of overwash, caused by waves that rush over a natural or artificial coastal barrier, leading to sand deposits on the leeside of the barrier. They often occur in nearshore zones, which extend from the low-tide line to beyond where waves influence sedimentation.
Barrier islands are part of the beach system, an area of unconsolidated material that stretches from the low-tide line to a region with permanent vegetation. They act as a critical buffer against coastal erosion and storms, protecting the bay/lagoon and inland areas. Additionally, they can occasionally form during periods of low relative sea level, known as lowstand, when more sediment is transported offshore.