Maphack is a generic term that refers to a method or third-party program that enables a user to see more of a level than intended by the developer. A maphacker is a user that deliberately executes such a method or program in the context of a relevant game, whilst maphacking is the act of such.
A common aspect of real-time strategy games is the player's partial limitation or complete inability to see beyond the visibility range of individual game objects that are under their ownership (typically units and structures), this concept is controlled by a mechanism known as the Fog of War. Maphacking usually enables the user to bypass this mechanism, either by removing it entirely and/or by rendering objects through the fog that would not normally be visible. In multiplayer modes, this allows for a distinct advantage against the other players that are subject to the intended settings. The advantage gained can be substantial, especially for the average real-time strategy games that rely on the rock-paper-scissors dynamic to balance out individual objects' varying strengths and weaknesses. Also called "Chams" or "Wallhacks" in most FPS games (such as Cross Fire, Team Fortress 2, and Call of Duty, to name a few) it can enable users to see "through" a wall, the outline of said player is generally seen in a gamma-intensive outline, such as neon greens, blues, reds, extremes (white/black), and pinks. Users can usually set what color to choose.