Significant other (abbreviated SO) is colloquially used as a gender-neutral term for a person`s partner in an intimate relationship{r|farlex} without disclosing or presuming anything about marital status, relationship status, or sexual orientation. Synonyms with similar properties include sweetheart, better half, spouse, domestic partner, lover, ....
[n] - a person with whom you have shared a long-term sexual relationship
According to Charles Horton Cooley, a person in our lives whose opinions matter to us and who is in a position to influence our thinking.