Two Young Blonde Girls Couple Taking Off Their Clothes

Significant other (or SO) is colloquially used as a gender-blind term for a person's partner in an intimate relationship without disclosing or presuming anything about marital status, relationship status, or sexual orientation, as it is vague enough to avoid offense by using a term that an individual might consider inappropriate (e.g. lover when they consider him a boyfriend, or girlfriend when they consider her a life partner).
In the United States the term is sometimes used in invitations, e.g., to weddings and office parties.
Its usage in both psychology and sociology is very different from its colloquial use. In psychology, a significant other is any person who has great importance to an individual's life or well-being. In sociology, it describes any person or persons with a strong influence on an individual's self-evaluation, which are important to this individual, as well as reception of particular social norms. This usage is synonymous with the term "relevant other" and can also be found in plural form, "significant others". This use of the term has become common in the UK in correspondence from hospitals, e.g., "you may be accompanied for your appointment by a significant other."