Music, often improvisational, originally developed around the turn of the 20th century by African Americans from elements of blues and ragtime, with syncopated rhythms and a European harmonic structure. Jazz consistently evolved throughout the 20th century with numerous phases, from a New Orleans style or swing, to "free jazz," for example, which emerged in the late 1950s. Therefore, jazz itself has no precise definition, however it has rarely been a completely composed music.
Source: Art & Architecture Thesaurus
http://vocab.getty.edu/page/aat/300266367
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