A person whom your parents know, from that job they had back in the day, whom they've never quite been able to shake off. This person will continually reappear at social gatherings involving other family friends, who all seem to just orbit around the periphery of your parents' lives for years and years, like a fly buzzing around your head that simply won't die.
Despite the ever-repetitive "look how tall you've gotten!"s and "I haven't seen you in ten years!"s from these strangers, you are required by unwritten social code to treat these people as if they were at least close enough to be a creepy uncle.
Mom: Our family friend Carl is coming to the barbecue on Saturday.
Mom: Oh you know, that guy I worked with 50 years ago, and we just happened to keep in touch. You've met him, he saw your birthing, remember?