In traditional terminology, a battle cruiser is any large armed cruiser (or warship) with battleship armament, that carries lighter armor than a battleship, in order to achieve a higher speed in a similar sized hull. This design was due to an early 20th century Earth naval theory that "speed is armor". The intended purpose of the battle cruiser was for scouting in advance of the main battle line, as well as screening the battle line from cruisers or smaller vessels while the battleships engaged their opposites in the enemy fleet. In practice, battle cruisers tended to be used to strengthen the main battle line because of their armament. This led to many disasters as they were not durable enough to engage battleships.
In spacecraft classification, the term battle cruiser is sometimes used synonymously with heavy cruiser.