Significant other (or SO) is a gender-blind, politically correct term to refer to a person’s partner in a romantic relationship without disclosing his or her personal or lifestyle choices or sexual orientation.

The first known occurrence of the term was in 1953 by U.S. psychiatrist, Harry Stack Sullivan, a former editor of the journal Psychiatry, in his work, The Interpersonal Theory of Psychiatry.

Its usage in both psychology and sociology is much 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 a person with a strong influence on an individual's self-evaluation as well as reception of particular social norms. This usage is synonymous with the term "relevant other."