Red dot cheat

Red dot is a multiplayer cheat which adds a red dot to the center of the screen, intended for use as a crosshair to aid aiming. This is considered unfair as it may provide an aim advantage to the cheating player. This is only useful in vanilla Doom, as almost all modern source ports have a 'crosshair' option, which can even be customizable.

The cheat usually takes the form of an IWAD modification.

Detection of this cheat involves visual observation of the player's game as it is played.

Source

 * This writeup includes text from Red Dot Doom2 cheat, used with permission.