Defender is a horizontally scrolling shooter video game developed by Williams Electronics in 1980 and released for arcades in 1981. The game is set on either an unnamed planet or city where the player must defeat waves of invading aliens while protecting astronauts. Development was led by Eugene Jarvis, a pinball programmer at Williams; Defender was Jarvis's first video game project and drew inspiration from Space Invaders and Asteroids. Defender was demonstrated in late 1980, before entering production in early 1981. It was distributed in Japan by Taito.

Shoot 'em ups are a sub-genre of action games. 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, while others allow a broader definition including characters on foot and a variety of perspectives.