A leitmotif (also spelled leitmotiv) is a recurring musical motif, associated within a particular piece of music with a particular person, place or idea. They are usually short melodies, although can also be chord progressions or even simple rhythms. Leitmotifs can help to bind a work together into a coherent whole, and also enable the composer to relate a story without the use of words, or add an extra level to an already present story.
The word is usually used when talking about dramatic works, especially opera, although leitmotifs are used instrumental pieces as well. The Symphonie Fantastique by Hector Berlioz is purely instrumental, but has a recurring melody representing the love of the central character. Berlioz himself called this an idée fixe. There is also some similarity with Tchaikovsky's use of motto themes - in his fifth symphony, for example, when one particular melody is representative of fate.
Richard Wagner is the composer most often associated with leitmotifs, and his operas make liberal use of them. His cycle of four operas, The Ring of the Nibelung, have certain leitmotifs which occur in more than one, even in all four, operas. However, leitmotifs had been used by other composers before him, most notably Carl Maria von Weber, who was probably the first to make extensive use of them. The first use of the word "leitmotif", however, was not until 1871, when critic F. W. Jähns used it to describe Weber's work. The term was apparently first used in reference to Wagner's music by H. von Wolzogen, the editor of the Bayreuther Blätter, who used it in 1887 when discussing Götterdämmerung. Wagner had called the musical phrases "Grundthema" (basic idea).
Since Wagner, the use of leitmotifs has been taken up by many other composers. In opera, Richard Strauss used the device in many of his works as well as in several of his symphonic poems. The Russian composer Sergei Prokofiev made heavy use of leitmotifs in his work Peter and the Wolf[?], a musical story with narration. In Peter and the Wolf, each character is represented by a specific instrument in the orchestra, and has its associated theme.
They are also very common in movie scores; consider for example the Imperial March associated with Darth Vader in the movie Star Wars. The television soap opera Dynasty also used musical themes for each character.
The word leitmotif has been used by extension to mean any sort of recurring theme, whether in music, literature, or somebody's life in general.