Fog of war
The strategy-game device that hides the parts of the map no friendly unit currently sees, and shows already-explored but unwatched areas as a dimmed memory of how they last looked.
The term comes from military theory — the uncertainty about the enemy's position and strength, often credited to Carl von Clausewitz's "Nebel des Krieges". In games it became a mechanic: the map starts black, exploration lifts the dark, but what you cannot currently see falls back into a half-light where the terrain remembers its shape while the enemy's movements stop updating. Empire (1977) had early versions; Warcraft and StarCraft made it standard in real-time strategy.
Technically it is a visibility grid laid over the map: each unit lights up the cells within its sight radius, usually through some form of line of sight, and a texture overlay dims the rest. Beyond the atmosphere, the fog is a load-bearing mechanic — it makes scouting valuable and ambushes possible. Cheating by lifting the whole fog is a classic way to wreck the balance of otherwise well-built games.