A battlecruiser is a large gunship, larger than a cruiser but smaller than a battleship. It is has the size and guns of a battleship but substantially thinner armor, which gave it greater speed. The idea was that the guns would allow it to take out destroyers, cruisers, and other smaller ships before they ever got into the range of their smaller guns, while its speed would enable it to escape enemy battleships.
In practice, this tactic was difficult to maintain. At the Battle of Jutland in World War I, the English battlecruisers engaged the German battleships before the arrival of the battleships of the English Grand Fleet, with disastrous results. The battlecruisers HMS Queen Mary and HMS Indefatigable exploded with the loss of all but a handful of their crews, and HMS Lion only survived by intentionally flooding one of her magazines. No English battleship was sunk during the battle. Thereafter, the Royal Navy de-emphasized battlecruisers. HMS Hood was launched in 1920, and was the last English battlecruiser to be built. Her lighter armor also proved a fatal weakness, as she exploded and sank in battle with the battleship Bismarck during World War II.
Other navies stuck with the battlecruiser concept somewhat longer. The US Navy aircraft carriers Lexington and Saratoga were built on battlecruiser hulls repurposed after the Washington Naval Treaty of 1922. The German Scharnhorst and Gneisenau were labelled battlecruisers, but they traded lighter armament (11-inch main guns) rather than thinner armor for speed.
Improved engine technology also worked against the battlecruiser formula. The ultimate limit on ship speed was drag from the water displaced (which increases as a cube of speed) rather than weight, so heavier armor only slowed World War II battleships by a couple of knots over their more lightly armored brethren. As it turned out, however, the carriers made both battleships and battlecruisers largely obsolete.