As fog of war (sometimes abbreviated as fow) is an optical concealment of parts of a map that the player has not direct vision of. In gaming context this may also refer to unexplored parts of the map, that are very often entirely covered, while classic fog of war (of already explored parts of the map) only conceals the actual situation in that part of the map, but not the looks and topography of the map without mobile objects. Very often the initial concealment of unexplored parts of the map are kept in black. Once you explored them, the actual fog of war is used as a greyish overlay, that appears, once there are no units or other means of uncovering that area. Here is an example, taken from the game Star Trek: Armada:

You can clearly see the unobstructed view on the right half. On the left quarter you can see the unexplored part of the map. In-between you find the fog of war covered area, showing the already discovered second Dilithium moon. Very similarly, the overview map also is designed in that fashion:

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