Video Games Encyclopedia

Light-gun shooter, also called light-gun game or simply gun game, is a shooter video game genre in which the primary design element is to simulate a shooting gallery by having the player aiming and discharging a gun-shaped controller at a screen. Light-gun shooters revolve around the protagonist shooting virtual targets, either antagonists or inanimate objects, and generally feature action or horror themes and some may employ a humorous, parodic treatment of these conventions. These games typically feature “on-rails” movement, which gives the player control only over aiming; the protagonist’s other movements are determined by the game. Games featuring this device are sometimes termed “rail shooters”, though this term is also applied to games of other genres in which “on-rails” movement is a feature. Some, particularly later, games give the player greater control over movement and in still others the protagonist does not move at all. On home computer conversions of light-gun shooters, mouse has been often an optional or non-optional replacement for a light gun.
Light-gun shooters typically employ “light gun” controllers, so named because they function through the use of light sensors. However, not all “light-gun shooters” use optical light guns, but some may also use alternative pointing devices such as positional guns or motion controllers. Mechanical games using light guns had existed since the 1930s, though they operated differently from those used in video games. Throughout the 1970s mechanical games were replaced by electronic video games and in the 1980s popular light-gun shooters such as Duck Hunt emerged. The genre was most popular in the 1990s, subsequent to the release of Virtua Cop, the formula of which was later improved upon by Time Crisis. The genre is less popular in the new millennium, as well as being hampered by compatibility issues, but retains a niche appeal for fans of “old school” gameplay.

Reference: Wikipedia