Structure built parallel to the shoreline and seaward of the beach designed to protect the beach and upland areas by causing waves to break and dissipate their energy before reaching the shore. Often used to construct small boat harbors.
The Ohio Department of Natural Resources has a good description of breakwaters, including the reasons why they are built, ways they are frequently laid out and some excellent photos at this page.
Breakwaters are constructed as both attached structures extending out from the shore and detached structures which are not attached to the shore. Ohio DNR has a separate page with description and pictures of detached breakwaters here.
For a more detailed discussion of breakwaters and other shoreline structures, including how they can negatively impact the movement of sand on beaches and destroy surf spots, see the larger article on Shoreline Structures