A person, or persons, whose presence at an event can be a determining factor as to whether or not you attend because the person's participation in the event would greatly diminish its enjoyability.
Vyatt: "Hey, some of us are getting together for a little outing, you in?"
Me: "Who's going?"
Vyatt: "Me, Barack, John, Hillary, Dennis..."
Me:"Hillay's a deal-breaker.."
Me: "You know, something came up at work..."