Its when you play DayZ, and log off in the middle of a fight, generally when in cover so the other player is still hunting for you, but wil never find you
(player 1 and his friend see player 2, and starts shooting)
(player 2 jumps behind a rock and logs off)
player 1: "Come on out!"
player 1's friend: "Don't bother, he's ghost logging"