significant other
A person with whom one is in a serious romantic relationship. Primarily heard in US. Since Kelly didn't have a significant other, she attended her friend's wedding alone.
your significant other
A person's significant other is their wife, husband, or the person they are having a relationship with. Poor Holly, she hadn't any significant other to bring with her. Well, we always work together, we live together — he's my significant other.
significant other
a person with whom someone has an established romantic or sexual relationship. 2001 Journal of Sex Research We asked: ‘Are you currently in a relationship? (Do you have a significant other, boyfriend, girlfriend, sexual partner, spouse, etc.?)’ and ‘If you answered “YES”, how long have you been in your current relationship?’