A monitor is a type of small warship that is neither fast nor strongly armored but carries disproportionately large guns. Monitors were used by some navies from the 1860s, during the First World War a
M15-class monitor
…The M15 class comprised fourteen monitors of the Royal Navy, all built and launched during 1915. The ships of this class were ordered in March, 1915, as part of the Emergency War Programme of…
USS Monitor, the first monitor (1861)
HMS Marshal Ney used a surplus 15-inch gun battleship turret.
Officers of a Union monitor, probably USS Sangamon, photographed during the American Civil War