Shore-perpendicular structures built at the sides of an inlet to maintain navigable waterways. They stabilize an inlet by intercepting the longshore transport of sand that would otherwise fill it in or cause the channel to shift position. Jetties are often confused with groins, but are more substantial structures, usually built in pairs.
Ohio DNR has a Web page with more information and photos of jetties.
For a more extensive discussion of jetties and other shoreline structures, please see the Shoreline Structures article