Reunion sex is really good sex in that you wouldn't go there again if the sex wasn't good when you were in the relationship with the ex. It's comfortable, no strings attached sex that should only happen once every 6 months after break-up.
2. Sex with a previous sexual encounter whom you believed or intended to be a one-night stand thus making it a two-night stand or more... aka Reunion Sex.
Guy B:"Oh, nothing, my ex came over."
Guy A:"You're not getting back together, are you?"
Guy B:"Hell no, we just had some reunion sex, that's all."
Girl A:"Hey girl, I saw you leave the bar last night with that guy. He was hot... did someone have a one-night stand?"
Girl B:"No... I mean we totally hooked up, but it wasn't the first time. We hooked up a few years ago, was definitely in need of some reunion sex."