Your mate, spouse, husband, wife, boyfriend, girlfriend, steady and/or lover. Used when you don't want to be too specific, or when the details are nobody's business.
Why's Roger always talking about his significant other? Why don't he just say girlfriend?
A significant other whose status is not created by a agreed-upon relationship status but by how much time is spent with them. Could also be considered a behavioral significant other.
Since he spent twice as much time with Karen as with any of the other girls he was dating, his friends called Karen his statistically significant other.