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Longman Dictionary of Contemporary English
significant other
noun
EXAMPLES FROM CORPUS
▪ In your case, the significant other is also a significant bother.
▪ So, how can you get useful feedback from your significant other, aside from signing up for couples' counseling classes?
▪ Your significant other must agree to provide comments in the most open and positive way possible.
Wiktionary
significant other

n. 1 One's romantic partner, regardless of marital status or gender. 2 A person with whom one has an important bond of some kind.

WordNet
significant other

n. a person (not necessarily a spouse) with whom you cohabit and share a long-term sexual relationship [syn: domestic partner, spousal equivalent, spouse equivalent]

Wikipedia
Significant Other

Significant Other is the second album by American rap rock/ nu metal band Limp Bizkit. Released in 1999 by Flip/ Interscope Records, the album saw the band expanding its sound from that of its debut album Three Dollar Bill, Yall, to incorporate further metal and hip hop influences. Significant Other was co-produced by Terry Date and Limp Bizkit. The album has sold at least 16 million copies worldwide.

During the band's live performance at Woodstock 1999, violence erupted during the album's song " Break Stuff". However, the controversy did not affect the album's success, as Significant Other received high commercial sales, peaking at number one on the Billboard 200. Critical reception to the album was favorable, with critics responding well to the album's unique sound and the band's performance, which was considered to be an improvement over the band's debut.