Because of the oddly eccentric nature of its whole being, jazz has enjoyed the questionable benefits of definitions that are either tautological--"Jazz music is any music played by jazz musicians"--or confusing to the layman--"Jazz is a matter of lip-technique." What can be said with confidence is that, whereas in more conventional musical areas the artist is fundamentally an executant expressing the findings of the creative mind of the composer, in jazz the performer is usually his own composer. Within the strict meaning of the term, there can be no such thing as a jazz theme, although of course some themes will lend themselves to the jazz idiom more readily than others. The customer, unable to acquire the recording of Brahms's Fourth Symphony conducted by Herbert von Karajan, would probably settle for someone else's recording of that work. But the buyer thwarted in his attempt to buy the jazz musician Duke Ellington's version of "Caravan" might well accept as a substitute anything else played by Ellington. That is why, in jazz, there is at least one truism that has always applied: the performer playing a theme always tries to make it sound not like itself but like himself.

[Click here for more on Jazz at Encyclopedia Britannica.com http://www.britannica.com]