The term for a close friend whom you used to hang out with often, but now see only rarely such as at birthday parties for mutual friends and other special social events. Often these encounters are awkward as mutual friends bring you together obligatorially, not voluntarily.
Friend 1: "Have you seen or spoken to Bob at all?"
Friend 2: "No, I haven't. We used to hang all the time, but kind of had a falling out and we don't talk all that much any more. He's just a calendar friend."