leitmotif

leitmotif \LYT-moh-teef\, noun:

1. In music drama, a marked melodic phrase or short passage which always accompanies the reappearance of a certain person, situation, abstract idea, or allusion in the course of the play; a sort of musical label.
2. A dominant and recurring theme.

Each actor to appear on stage is accompanied by a musical phrase on the drum -- a sort of leitmotif to characterize an emotion, much like a Wagnerian drama.
-- Eleanor Blau, "Connecticut's Shakespeare", New York Times, July 9, 1982

One theme had recurred so frequently in these conversations that it had become the leitmotif of the trip.
-- Jack F. Matlock Jr., Autopsy on an Empire

As is so often the case in a crazy household . . . guilt becomes a leitmotif.
-- Frederick Busch, "My Brother, Myself", New York Times, February 9, 1997

Leitmotif (also spelled leitmotiv) is from German Leitmotiv, "leading motif," from leiten, "to lead" (from Old High German leitan) + Motiv, "motif," from the French. It is especially associated with the operas of German composer Richard Wagner.