With explanation in brackets, if deemed appropriate to the nature of the text.

A leitmotif /ˌlaɪtmoʊˈtiːf/ is a "short, constantly recurring musical phrase"[1] associated with a particular person, place, or idea. It is closely related to the musical concepts of idée fixe or motto-theme.[2] The term itself is an anglicization of the German Leitmotiv, literally meaning "leading motif", or perhaps more accurately, "guiding motif". A musical motif has been defined as a "short musical idea ... melodic, harmonic, or rhythmic, or all three",[3] a salient recurring figure, musical fragment or succession of notes that has some special importance in or is characteristic of a composition: "the smallest structural unit possessing thematic identity."[4]
http://en.wikipedia.org/wiki/Leitmotif

leitmotif, German Leitmotiv (“leading motive”), a recurring musical theme appearing usually in operas but also in symphonic poems. It is used to reinforce the dramatic action, to provide psychological insight into the characters, and to recall or suggest to the listener extramusical ideas relevant to the dramatic event. In a purely musical sense the repetition or transformation of the theme also gives cohesion to large-scale works.

The term was first used by writers analyzing the music dramas of Richard Wagner, with whom the leitmotif technique is particularly associated. They applied it to the “representative themes” that characterize his works. The close thematic musical structure of his dramas, from Der Ring des Nibelungen onward, including Tristan und Isolde and Die Meistersinger, demands skillful contrivance and keen intelligence in order to make the themes work satisfyingly in a symphonic way and at the same time enrich the dramatic events.

The leitmotif has two distinct dramatic functions, which may operate separately or together: one is allusion (to dramatic events), the other transformation, or continual modification of the theme. Both were used long before Wagner. Wolfgang Amadeus Mozart’s four-measure phrase “Così fan tutte” (“Thus do they all”), in his opera of the same name, is allusive, but it appears as a recurrent motto rather than as a true leitmotif. Another early example of such allusive use is in Carl Maria von Weber’s opera Der Freischütz (The Freeshooter, or, more colloquially, The Magic Marksman), when Max hesitates to descend into the wolves’ glen and the orchestra echoes the mocking chorus that had teased him in the first act.
http://www.britannica.com/EBchecked/topic/335529/leitmotif