From The Art and Popular Culture Encyclopedia
Incidental music is music in a play, television program, radio program, video game, film or some other form not primarily musical. The term is less frequently applied to film music, with such music being referred to instead as the "film score" or "soundtrack."
Incidental music is often "background" music, and adds atmosphere to the action. It may take the form of something as simple as a low, ominous tone suggesting an impending startling event, or, to enhance the depiction of a story-advancing sequence, such as its use in the film The Insider. It may also include pieces such as overtures, or, music played during scene changes, or at the end of an act, immediately preceding an interlude, as was customary with several nineteenth-century plays. It may also be required in plays that have musicians performing on-stage.
The use of incidental music dates back at least as far as Greek drama. A number of classical composers have written incidental music for various plays, with the more famous examples including Ludwig van Beethoven's Egmont music, Franz Schubert's Rosamunde music, Felix Mendelssohn's music for A Midsummer Night's Dream, Georges Bizet's music for L'Arlésienne, and Edvard Grieg's music for Henrik Ibsen's Peer Gynt. Parts of all of these are often performed in concerts outside the context of the play. Vocal incidental music, which is included in the classical scores mentioned above, should never be confused with the score of a Broadway or film musical, in which the songs often reveal character and further the storyline. Since the score of a Broadway or film musical is what actually makes the work a musical, it is far more essential to the work than mere incidental music, which nearly always amounts to little more than a background score; indeed, many plays have no incidental music whatsoever.
The genre of incidental music does not extend to pieces designed for concert performance, such as overtures named after a play, for example, Beethoven's Coriolanus overture, or Tchaikovsky's Romeo and Juliet fantasy-overture.