Significant other (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 he or she considers him or her a boyfriend or girlfriend. Other synonyms include sweetheart, better half, spouse, domestic partner, or lover, when he or she considers him or her a soulmate or life partner).
In the United States the term is sometimes used in invitations, e.g., to weddings and office parties. 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."
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 concept. Although the influence of significant others on individuals was long theorized, the first actual measurements of the influence of significant others on individuals were made by Archie O. Haller, Edward L. Fink and Joseph Woelfel at the University of Wisconsin.
Haller, Fink and Woelfel are associates of the Wisconsin model of status attainment. They surveyed 100 Wisconsin adolescents, measured their educational and occupational aspirations, and identified the set of other individuals who communicated with the students and served as examples for them. They then contacted the significant others directly and measured their expectations for the adolescent's educational and occupational attainments, and calculated the impact of these expectations on the aspirations of the students. Results of the research showed that the expectations of significant others were the single most potent influences on the students' own aspirations. This usage is synonymous with the term "relevant other" and can also be found in plural form, "significant others".
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.
First use 
The first known occurrence of the term was in 1953 by U.S. psychiatrist, Harry Stack Sullivan, in his posthumously published work, The Interpersonal Theory of Psychiatry. While the term has since generated greater familiarity, use of the term peaked in the late 1980s to mid-1990s and has generally declined since then in favor of other terminology as deemed appropriate. However, because much of the other terminology is somewhat cumbersome, the use of the term "sigo" is beginning to gain some popularity.
The phrase was popularised by Derek Trotter in Only Fools and Horses, who used the phrase a number of times when referring to his long-term partner, Raquel Turner. Use of the phrase was often received with laughter from studio audiences, due to Del's oblivious belief that it had affectionate implications.
See also 
- Anon. "Significant other". The Free Dictionary. Farlex Inc. Retrieved 25 January 2010.
- Haller, A. & Woelfel, J. (1972) Significant others and their expectations: Concepts and instruments to measure interpersonal influence on status aspirations Rural Sociology, 37(4), 591-622 doi:10.1177/0049124190019001006
- Woelfel, J. & Haller, A. (1971) Significant others: The self-reflexive act and the attitude formation process American Sociological Review American Sociological Association: 36(1), 74-87
- Sullivan, Harry Stack (1953). The Interpersonal Theory of Psychiatry. New York: W.W. Norton & Company, Inc. ISBN 0-393-00138-5. OCLC 204035. page 9?