Crosshair is a player aid feature for indicating where the player's weapon is aimed at. Unlike most newer first-person shooters, the original Doom games did not have a crosshair and instead had the player's equipped gun centered on the screen as the sole aim indicator. However, after the Doom source code release, many source ports added crosshair support into the game, which, at the same time, serves as a companion feature for the popular source port additions of looking up and down and the related option for disabling autoaim. Doom 3, in turn, is similar to other shooters of its time, having equipped guns located at the corner of the screen with a crosshair for aiming.
Before source ports, several PWADs implemented a primitive crosshair setup via simple graphical mods, adding a crosshair into the player weapon sprite itself. A famous example of this is the Cryogun in Hacx.