Leitmotif
The word “leitmotif” is particularly associated with the operas of Richard Wagner.  The leitmotif helps to make the story dramatic and bind it together, because it makes the music easier to understand.  Sometimes a leitmotif will change during an opera as the character changes.  Various dramatic effects can be made with leitmotifs. For example, a leitmotif might be played before a character comes onstage, so the audience will know who is coming before the actor can be seen.  Three examples of leitmotifs from Wagner's Ring Cycle  Der Ring des Nibelungen  are: the leitmotif for the chief god Wotan (a person), the leitmotif for the Tarnhelm, the invisibility helmet, (a thing), and the leitmotif for the Renunciation of Love (an idea).