How do you define "Significant Other?"

Over on Sheroes, there have been a few threads on related topics. Pictures of your S.O. and how many have you had and other such things.

As I read responses, it seemed to me that folks had radically different ideas of what a significant other entails. So, I ask you, what makes somebody your significant other, not merely the person you are dating or your boy/girlfriend.

What makes it significant?!