In film music (and classical music, especially opera), a leitmotif is “a short, constantly recurring musical phrase, that is associated with a particular person, place, or idea.”
The most famous user of leitmotif in 21st century Hollywood would have to be composer John Williams in the seven Star Wars scores (1977-ongoing)
(for more on the leitmotifs of Star Wars see: The Empire Strikes Back or, Everyone has a Theme! Part One: Leitmotif and “The Imperial March” and The Empire Strikes Back Part Two!: Han and Leia in Love and Yoda!! )
Leitmotifs can be found in many films, for example, in the Star Trek franchise there is a popular theme known as the “Enterprise motif,” this is the fanfare of rising fourths that occurs almost every time the Enterprise appears on the screen (this is especially true in the original series and in the Next Generation films).
The concept of leitmotif (which roughly translates to “leading motive”) predates the creation of film by several decades and is closely associated with the late-Romantic composer Richard Wagner (though Wagner did NOT invent the concept himself as some have claimed)
Wagner’s operatic music had a HUGE influence on modern film music
In Wagner’s famous cycle of operas known as Der Ring des Nibelungen (The Ring of the Nibelungs, aka “The Ring Cycle”), Wagner created an entire series of leitmotifs to represent specific characters or themes in the story.
For example, the hero Siegfried is represented by a leitmotif known as “Siegfried’s Horn Call,” seen here below:
Another important motif represents the god Wotan’s spear:
Other motifs represent: Fire, The Rhine, The Ring, and Sleep
Whatever the context, leitmotifs are an integral part of a film score (when they are used), and they provide an interesting connection to the world of 19th century opera.