To masturbate or "whack off", whilst wearing a balaclava in the same room as your mate while he has sex with a girl. Often the "Phantom Batter" is hidden under a desk, under the bed or behind the curtains. Ultimately, the "Batter" needs to be hidden, all lights off and making sure to be very careful about how much noise he makes whilst rubbing one out.
By definition, its fine if your mate knows your in the room, but its only a "phantom bat" if the girl has no idea of the Phantoms presence, otherwise your just a broski having a toss.
Trev: "Michael was a Phantom Batter last night while i was giving it to Susan !"
Jason: "Haha, for real ?"
Michael: "Yeh bro, I was hiding under Trev's desk with a balaclava on ! She totally never saw me"