Serious Relationship?

The topic comes up in every dating relationship, but I am often confused by the subject. So my question to you is this: What does “serious relationship” mean to you? To me, a “serious relationship” is always exclusive. The inverse, however, is not necessarily true (i.e. a “non-serious relationship” isn’t always “open”). Since I guess I am on the open market, I figure I should get a grip on the subject.