In literature and film, a subplot was a storyline which was secondary to the main plot.
The novel Hotel Royale had a subplot about a woman planning to murder her husband with the help of an older man. (TNG: "The Royale")
When Brad Boimler, in the role of "Bucephalus Dagger", avoided discussing Dagger and Helena Gibson's past romantic issues, Beckett Mariner protested that he can't just cut out the romantic subplot. (LD: "Crisis Point 2: Paradoxus")