A breakwater is a permanent structure constructed at a coastal area to protect against tides, currents, waves, and storm surges). Breakwaters have been built since Antiquity to protect anchorages, helping isolate vessels from marine hazards such as wind-driven waves). They are designed to reduce the intensity of wave action in inshore waters and thereby provide safe harborage). Breakwaters may also be small structures designed to protect a gently sloping beach to reduce coastal erosion; they are placed 100–300 feet (30–90 m) offshore in relatively shallow water).

Breakwaters can be either fixed or floating, and impermeable or permeable to allow sediment transfer shoreward of the structures, the choice depending on tidal range and water depth). They usually consist of large pieces of rock (granite) weighing up to 10–15 tonnes each, or rubble-mound). Breakwaters may be connected to land or freestanding, and may contain a walkway or road for vehicle access).

Types of breakwaters include vertical wall breakwater, mound breakwater, and mound with superstructure or composite breakwater). A breakwater structure is designed to absorb the energy of the waves that hit it, either by using mass (e.g. with caissons), or by using a revetment slope (e.g. with rock or concrete armor units) ).

Breakwaters can be placed attached to the shoreline as headlands or submerged near the shoreline as sills. They allow for the accretion of sediment between the structure and the shoreline, potentially stabilizing wetlands and providing shelter for new intertidal marsh habitat. However, this may impede longshore transportation of material leading to downdrift erosion as well as form intertidal marsh not appropriate for the location, replacing natural sandy beach habitat.

In summary, a breakwater is a structure built offshore or along the shoreline to reduce the intensity of wave action in inshore waters and provide safe harborage. It can be either fixed or floating, and impermeable or permeable to allow sediment transfer shoreward of the structures. Breakwaters can be attached to the shoreline as headlands or submerged near the shoreline as sills.