Shoot 'em up is a subgenre of the shooter genre of video games. In a shoot 'em up, the player character moves forward automatically, often in a flying vehicle such as a spacecraft or aircraft, shooting large numbers of enemies while dodging obstacles. There is no consensus as to which design elements compose a shoot 'em up. Some restrict the definition to games featuring spacecraft and certain types of character movement; others allow a broader definition including characters on foot and a variety of perspectives. Shoot 'em ups call for fast reactions and for the player to memorize levels and enemy attack patterns. "Bullet hell" games are a relatively recent subgenre that features overwhelming numbers of enemy projectiles, often in visually impressive formations.