Shoot 'em up (also known as shmup or STG) is a subgenre of the shooter genre of video games. In a shoot 'em up, the player character engages in a lone assault, often in a spacecraft or aircraft, shooting large numbers of enemies while dodging their attacks. 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 feature overwhelming numbers of enemy projectiles.