USS Monitor was an ironclad warship commissioned by the United States Navy in 1861 and completed in 1862. She is most famous for her participation in the first-ever naval battle between two ironclad warships, the Battle of Hampton Roads on March 9, 1862, in which Monitor fought the ironclad CSS Virginia of the Confederate States Navy. She was also noted for containing several innovations, including the rotating turret, which led to future changes in shipbuilding. She was the first in a long line of Monitor-type U.S. warships and the term "monitor" came to describe a broad class of shallow-draft craft.
Ironclads were only a recent innovation, started with the floating ironclad batteries used in the Crimean War and the 1859 French battleship La Gloire. Afterwards, the design of ships and the nature of naval warfare changed dramatically.