Two Young Blonde Girls Couple Taking Off Their Clothes

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."
In social psychology a significant other is the parent, uncle/aunt, grandparent, or teacher—the person that guides and takes care of a child during primary socialization. The significant other protects, rewards, and punishes the child as a way of aiding the child's development. This usually takes about six or seven years, and after that the significant other is no longer needed, the child moves on to a general other which is not a real person, but an abstract notion of what society deems good or bad.