A cutlass is a type of saber, a slightly curved sword. Many sailors — and pirates — carried cutlasses on ships during the 17th century.
The cutlass was considered a "naval sword" partly because it was extremely useful on board a ship. A sailor or pirate could use a cutlass to cut thick rope and wood, and its size made it safer in tight quarters than a longer sword might be. Although they weren't used much after the 18th century, the cutlass was still an official weapon of the U.S. Navy until 1949. The Latin root means "knife."