What, precisely, is "fog of war"?

[TheDawg]

I offer an explanation not game based, but sourced from my work as a.... govt employee.

I hope it can provide a useful comparison to what actually happens in game.

I understood the concept of the phrase in real life, and google showed me the movie about that guy, but comparisons to RL and this game of mind since 2001 typically...is close, but not accurate, LOL!!
More curious as to exactly what it meant in OlegsWorld®