When actors engage in a romance for the run of a show.
Once the run is over so is the romance.
The term originated in the theater and moved to movies and scripted television and then reality television.
The term has now moved to the populace to discribe any contrived romance.
> Dude 1: I didn't know Anthony and Cicely were dating.
> Dude 2: It's just a showmance, Cicely is just dating Anthony just to make Edgar jealous.
> Dude 1: I didn't know Anthony and Cicely were dating.
> Dude 2: They're not. It's just a showmance, Anthony is gay.