Video Games Encyclopedia

Shoot ’em ups (also known as shmups or STGs) 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.
The genre’s roots can be traced back to earlier shooting games, including target shooting electro-mechanical games of the mid-20th-century, but did not receive a video game release until Spacewar! (1962). The shoot ’em up genre was established by the hit arcade game Space Invaders, which popularised and set the general template for the genre in 1978, and has spawned many clones. The genre was then further developed by arcade hits such as Asteroids and Galaxian in 1979. Shoot ’em ups were popular throughout the 1980s to early 1990s, diversifying into a variety of subgenres such as scrolling shooters, run and gun games and rail shooters. In the mid-1990s, shoot ’em ups became a niche genre based on design conventions established in the 1980s, and increasingly catered to specialist enthusiasts, particularly in Japan. “Bullet hell” games are a subgenre of shooters that features overwhelming numbers of enemy projectiles, often in visually impressive formations.

Reference: Wikipedia