Fog of War

Fog of War in the Warcraft RTS games refers to any area or section of the map that is covered by a grayed area. The gray areas covered by Fog of War indicate certain parts of the map that have been explored at some point during the game but only the terrain and any buildings standing at the time are known in these locations. Any units or newly placed buildings may still be hidden in the Fog of War area since the last time it was explored there. To reveal any new units or buildings, the player must once again explore the areas covered by Fog of War.

Fog of War is different from areas of the map which remain unexplored and are represented by a blackened shroud.

In Warcraft II
The Fog of War covers everything that is not within the sight range of units or buildings. When viewing the map, black areas have not yet been explored, while grayed areas have been explored but are not currently within sight range of owned or allied units or buildings. Although terrain can be seen through the grayed areas, enemy troop movements are hidden.

In Warcraft III
In Warcraft III: Reign of Chaos and Warcraft III: The Frozen Throne, the Fog of War is affected by the Day and Night cycle that exists in-game.

The Day/Night Cycle affects the sight range of units and buildings. Units and buildings see more during the day and less at night. This makes scouting during the day and troop movement during the night ideal. The Night Elves can research Ultravision which allows them to see just as well during the Night.

World of Warcraft
New tech was created for the Kil'jaeden Tomb of Sargeras encounter, called Fog of War internally. The players have limited vision so they have to use the help of their allies to make their way through that phase.

Fog of War later arrived as part of Island expeditions. The mechanic allowed for teams of three players to compete at PvE objectives, with reduced awareness of the other team's progress across the map until they came upon each other.