A relationship in which two people agree that they want to be together, but can't exactly promise that they won't see other people too. Basically, to have it all: a significant other and the freedom to hook up with other people. Common during college for many post-high school relationships.
"Are you still with Joe?"
"Well, we're in a open relationship now. And it's don't ask, don't tell."
"Are you okay with that?"
"You know, it IS college, and there are so many cute guys out there..."