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.
If you've seen peter and the wolf, or watched any good warner brothers cartoons, you've seen a leitmotif in action.
What's your leitmotif? I imagine mine to be a soft-playing brass... french horn and baritone, lapsing from a soft rich set of tones to a goofy, almost circus-like fanfare. (how's that for a roundabout conceit?)