Unresolved Sexual Tension
|See also:||RST, Eye Sex, Pining|
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UST is an abbreviation used to refer to unrequited sexual tension between two characters or unresolved sexual tension between the same, depending on the context.
The former involves a character undergoing angst (and possibly a lot of pining) at the reality of another character not sharing their feelings, and the latter, a character or characters expressing such tension in an alternative way, such as anger, denial, or even 'kisses that don't go anywhere'. Such tension can be in the realm of a het-oriented fanwork or a slash-oriented fanwork.
The term was supposedly coined by a fan named LAG. 
In The X-Files fandom, UST most commonly stands for unresolved sexual tension and often led to RST, or resolved sexual tension. For many Mulder/Scully shippers, UST was a favorite device both in fic and on screen. UST was such a big part of the fandom that the Gossamer archive suggests using UST as a possible keyword for fic headers.
- "Unresolved Sexual Tension--refers to the undercurrent of romantic and sexual attraction between Mulder and Scully, something neither of them will acknowledge. UST does =not= (necessarily) have to be Resolved, at least not in the near future. This definition is not a Final Word, but to give people a common definition for basis of discussion. First used by LAG" -- Weekly Post: Acronyms explained! , November 18, 1994