origin:  German Leitmotiv, leiten= to lead + Motiv= motive
Imagine that Darth Vadar has just entered the room...what do you hear inside your head?
If it's something like the video below, then you already know what a leitmotif is! It's a melody or "phrase" (words to describe verbal voice patterns are frequently applied to parts of music as well) that accompanies the appearance of an individual, concept, or situation.
This idea was well-used by Wagner in his operas, hence the term "space opera" for Star Wars! You can find them in many films as well, such as Gone With The Wind, Jaws, Lord of the Rings, or Dr. Zhivago. Today leitmotifs are also commonly used in children's shows to help them follow ideas and plots (just as in opera): He-Man having music that accompanies his transformation, battles, and individual characters as does Sailor Moon (and many others). You may have also spotted it in video games -- the mere mention of Mario Bros has been known to trigger the theme inside the heads of players!
A leitmotif may also be applied to literature, as a theme which keeps reoccurring, literacy itself becomes one in the work of Dickens; a character who can read plays a very different part than one who cannot.