A leitmotif is a "short, constantly recurring musical phrase" associated with a particular person, place, or idea. It is closely related to the musical concepts of idée fixe or motto-theme. 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", 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."In particular, such a motif should be "clearly identified so as to retain its identity if modified on subsequent appearances" whether such modification be in terms of rhythm, harmony, orchestration or accompaniment. It may also be "combined with other leitmotifs to suggest a new dramatic condition" or development. The technique is notably associated with the operas of Richard Wagner, although he was not its originator and did not employ the word in connection with his work.

Although usually a short melody, it can also be a chord progression or even a simple rhythm. 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 to add an extra level to an already present story.

By association, the word has also been used to mean any sort of recurring theme, (whether or not subject to developmental transformation) in literature, or (metaphorically) the life of a fictional character or a real person. It is sometimes also used in discussion of other musical genres, such as instrumental pieces, cinema, and video game music, sometimes interchangeably with the more general category of theme. Such usage typically obscures the crucial aspect of a leitmotif—as opposed to the plain musical motif or theme—that it is transformable and recurs in different guises throughout the piece in which it occurs.