A breastwork monitor was a type of ship originated by Sir Edward Reed, the Chief Constructor of the Royal Navy between 1863 and 1870. The term "monitor" was directly derived from the American ship of that name, the USS Monitor designed by John Ericsson which served in the American Civil War. The American ships were very stable, and difficult to damage by gunfire, because of their very low freeboard. This, however, caused them to behave, some said, as a "half-tide rock", with the ever present risk of being swamped in a sea should water gain access to the interior through hatches, turret bases or other openings in the deck.
Reed proposed to overcome this risk by the addition of an armoured breastwork. This was an armoured superstructure of moderate height (7 feet (2.1 m) in HMVS Cerberus), centrally placed on the ship and containing within its armoured circumference the gun turrets, bridge, funnels and all other upper deck appurtenances needed to operate the ship. The presence of this breastwork allowed the ship to operate without fear of being flooded by waves breaking over the deck, and allowed the main armament to be positioned at a greater height than in the American monitors, gaining thereby greater command and range, while preserving the defensive advantage of low freeboard.
Reed's concept was copied by other designers and breastwork monitors served in the French and Imperial Russian Navies, among others.