"UST" is an acronym for "Unresolved Sexual Tension." For example, in FK's "Only the Lonely," you may interpret Nick's behavior toward Natalie as brotherly, as he claims, or you may interpret it as influenced by UST -- a strong attraction that is going unacknowledged by the principals. (Or both! Nick's complicated. ~grin~)

I've always understood UST to be primarily about emotions -- longing, pining, imagination, reflection, self-sacrifice -- rather than about sex, so I was a little surprised by Sharpest_Asp's note, above in this thread, that she's familiar with the term as applying primarily to physicality.

"Pre-slash" used to tell the reader: "This may look like a gen story, but it's actually not about a platonic friendship; it's about a budding romance." Generally, the term seemed to me to identify stories about two canonically heterosexual characters on their way toward a homosexual relationship. (This was from the days when there were next to no canonically homosexual characters in any public medium, so subtext was a constant question.)

I don't know whether the term is still used that way, or whether I ever truly understood it in the first place, which is why I'm asking.