H. L. Hunley, also known as the Hunley, CSS H. L. Hunley, or CSS Hunley, was a submarine of the Confederate States of America that played a small part in the American Civil War. Hunley demonstrated the advantages and dangers of undersea warfare. She was the first combat submarine to sink a warship (USS Housatonic), although Hunley was not completely submerged and, following her attack, was lost along with her crew before she could return to base. Twenty-one crewmen died in the three sinkings of Hunley during her short career. She was named for her inventor, Horace Lawson Hunley, shortly after she was taken into government service under the control of the Confederate States Army at Charleston, South Carolina.

A submarine is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely operated vehicles and robots, as well as medium-sized or smaller vessels, such as the midget submarine and the wet sub. Submarines are referred to as boats rather than ships irrespective of their size.