A monitor was the class of relatively small warship which was neither fast nor strongly armored but carried disproportionately large guns. They were used by some navies from the 1860s until the end of World War II, and saw their final use by the United States Navy during the Vietnam War.
The original monitor was designed by [Swedish-American] John Ericsson in 1861 who named it the USS Monitor. It was the first ironclad as well as the first to use gun turrets. They were designed for shallow waters and served as coastal ships. The term “monitor” also encompassed more flexible breastwork monitors, and was sometimes used as a generic term for any turreted ship.