Fog of war

Fog of war is a military concept that has been adapted to video games which describes the uncertainty of what is going on outside the range of an army's surveillance — usually areas within the enemy's territory or areas too remote to effectively survey. In video games, "fog of war" is used to more accurately simulate the real world aspects of military combat and refers to the concept as well as the game mechanic which enforces it.
Fog of war has been implemented in a number of ways over the years with some methods being more effective than others. It is especially common in combat-based strategy video games.
History
Fog of war in gaming saw its start on board games where pieces with information on only one side allowed players to see their own information, while obscuring it from their opponents. Stratego made use of this. This was about the best a board game can do without resorting to a cumbersome amount of pieces and movement, so it didn't improve until the advent of video games.
However, the vast majority of early combat video games were more like electronic versions of miniature games which revealed the entire map to all players from the beginning, and every player knew the position of all of their opponent units all the time. Just like with miniatures, this is a complete lack of fog of war.
Later games would start the game with the map initially undiscovered, often signified by it being covered in blackness. The player would have no idea what the surrounding landscape was like or what their opponents were up to in those areas until the player sent one of their own units on reconnaissance to explore the void. However, once explored, the player's ability to view the area remained unrestricted, even if units no longer remained there. Empire, released in 1977, is the first game known to use a system like this, but, until the 1990s, few games followed its lead. This form of fog of war slowly uncovers the map, which is a nice progression, but it isn't very realistic due to real-time surveillance of unlimited range.
Panzer General, released in 1994, uses a more realistic system of range-based fog of war. In the game, every unit has a visual range, and the player can only see enemy movements that are occurring within the visual range of their units. However, in Panzer General, the player initially knows the complete terrain of the world, even those areas in their opponent's section of the map.
In 1995, WarCraft II: Tides of Darkness merged the two aforementioned forms. The entire map is initially undiscovered, and moving a unit there will reveal the landscape, however, if your units leave the area, the area will be grayed-out. You will be able to see the landmarks of the area, like forests and lakes, but you won't see any enemy units moving through the area or building structures in the area.