A relationship in which people who are more than 'more than friends' but not lovers. It is a step in the right direction for a couple looking for a genuine relation, and scary for those who fear commitment (or AC power).
Roommate 1: Where is Jeff tonight?
Roommate 2: He is visiting his serious friend... they are serious friends, dude....he'll back in the morning.
Roommate 1: Wow that's serious.