A person who is not quite a friend yet but more than an acquaintance. You take part in light-hearted chat and would most probably only talk to them if you were with a group of people; the same goes for hanging out with them. You also might say Hi to them when meeting in the street, but you wouldn't stop and chat.
Dave: Can you get Fred to lend me Spiderman 3 on DVD? I don't really know him that well.
Gary: Hmmm... I'm not sure if I can ask him yet, we're really only social-colleagues at the moment. Ask Kyle though, he was at friend level last time I checked.