Subplot
In fiction, a subplot is a secondary strand of the plot that is a supporting side story for any story or the main plot. Subplots may connect to main plots, in either time and place or in thematic significance. Subplots often involve supporting characters, those besides the protagonist or antagonist.
Subplots are distinguished from the main plot by taking up less of the action, having fewer significant events occur, with less impact on the 'world' of the work, and occurring to less important characters.
In screenwriting, a subplot is referred to as a "B story" or a "C story," etc.
Examples
In William Shakespeare's play Henry IV, Part II, the main plot concerns Henry's growth from "Hal" the prince to "Henry" the king and the reconquest of French territory. A subplot, however, concerns Falstaff's participation in the battles. Falstaff and Henry meet at several points, and Falstaff is a familiar of Henry's, but otherwise his plot and Henry's do not mix. Even though the plots may be thematically connected, they are not connected in action.