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. It may also include pieces such as overtures, 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 Midsummer Night's Dream music, Georges Bizet's L'Arlésienne music, 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 Coriolan Overture (written for Heinrich Joseph von Collin's tragedy), or Tchaikovsky's Romeo and Juliet fantasy-overture.
An overture is incidental music that is played usually at the beginning of a film, play, opera, etc., before the action begins. It may be a complete work of music in itself or just a simple tune. In some cases it incorporates musical themes that are later repeated in other incidental music used during the performance.
Theme song 
A theme song is a work that represents the performance and is often played at the beginning or end of the performance. Elements of the theme may be incorporated into other incidental music used during the performance. In films, theme songs are often played during credit rolls. A love theme is a special theme song (often in various modified forms) that accompanies romantic scenes involving the protagonists of a performance.
Theme songs are among the most works of incidental music that are most commonly released independently of the performance for which they were written, and occasionally become major successes in their own right.
An underscore is soft, unobtrusive background music that accompanies the action in a performance. It is usually designed so that spectators are only indirectly aware of its presence. It may help to set or indicate the mood of a scene.
A stinger is a very brief instant of music that accompanies a scene transition in a performance. Often the stinger marks the passage of time or a change in location. Stingers were used frequently in the American television series Friends (as one example) to mark scene transitions involving the passage of time or a change of location. Similar techniques are commonly used in many American sitcoms.
Short sequences of recorded music called loops are sometimes designed so that they can be repeated indefinitely and seamlessly as required to accompany visuals. These are often used as background music in documentary and trade films.