Ghosting, or "to ghost" is The act of one or more dead players spectating an alive enemy player, with the intent of revealing his or her actions or location to their alive comrades, giving them an unfair situational advantage. This is achieved usually through a voice chat program, such as Ventrillo, or an IRC client.
Ghosting may also be referred to as as "Venting" in popular games like Counter-Strike. The term derives from the most popular method of ghosting in Counter-Strike, which is to use the program Ventrillo, "venting" is short for "ventrillo-ing" meaning Using Ventrillo to ghost
"WTF? that douche nozzle is totally ghosting with one of this friends. He was shooting at me, before he even saw me and he seems to know every location of our teammates!"
"Wow, this guy is predicting the future......I think he might be Venting with his guild"