Subplot
A narrative line running parallel to the main plot, enriching it, contrasting it or giving it air.
A subplot unrolls a narrative line parallel to the main quest: a love story inside a thriller, a secondary character's trajectory, a family conflict beneath the investigation. It gives the novel its thickness of world — life doesn't stop because a quest begins.
Its functions are precise: pacing relief (alternating fronts), thematic contrast (a subplot handling the same theme differently throws it into relief), feeding the main plot by pouring its consequences back in. The golden rule: every subplot must cross the main line and bend it; otherwise it's a second novel squatting in the first.
Example
In a detective novel, the investigator's crumbling marriage — until the two threads knot together.