A friend in a friendship bond by a shared situation.
Simply speaking, it's the kind of friendship which gets cut after you leave your school or job.
Ever wondered why you lost contact with most your high school buddies; guys you hanged with for hours and considered close friends? You probably still remember them as witty and interesting, so where has the charm gone? And when, eventually, you meet them on the street by chance (and that's not a neglectable chance in my country), why do you talk about the weather?
It's hard to admit but your friendship was based on being stuck in the same place, and if you haven't kept contact - that's probably a sign there wasn't much more to it. Meeting a few years later, you'd notice you have practically nothing to talk about, having no common ground for conversation (the daily events, which you no longer share) - and for all concerns, you're strangers. It's hard to keep those friendships alive after leaving the situation and it's even harder to revive them after distancing. Thus, as you move on, don't feel committed: those people merely accompanied you on one of your life's journeys and now your paths parted.
On a personal note, I know someone who's about to finish high school and, being aware of this phenomenon, is afraid of losing touch with all the people who now seem dear. I can only symphatize and be glad I left it to time to determine which friends I'll stick with. My next situation change is few years away, and by then I'd surely have those doubts too... Will you recognize me; call my name or walk on by?