Like the term jazz itself, a precise definition of jazz song is elusive. One way to think about it is that a jazz song is anything sung by a jazz singer, since the term 'jazz' usually refers to a style of performance rather than to a method of composition. A jazz song might have lyrics, but not necessarily. It might be a vocalese performance, with lyrics written and sung to a pre-existing instrumental solo (i.e. the work of King Pleasure, Eddie Jefferson, or Lambert, Hendricks & Ross). It could be a "standard" song taken from what is often referred to as the American Popular Song Book. "Standards" come from a variety of sources including popular songs by Tin Pan Alley composers from the beginning of the 20th century, American popular stage, or Hollywood movie musicals. In addition, a jazz song can be based on blues, pop songs, a poem or any instrumental in the jazz repertoire. It might incorporate elements of scat, or nonsense syllables sung in horn-like fashion. Or it might be a contrefact; a variation on pre-existing chord changes, such as "Donna Lee," a song composed by Miles Davis using the chord changes of "(Back Home Again in) Indiana", or "The Flintstones Theme" which is one of many songs based on the chord changes of "I Got Rhythm" by George and Ira Gershwin.
Occasionally original jazz compositions speak directly to political and social issues of the day such as "Strange Fruit" by Billie Holiday, "We Insist (Max Roach's Freedom Now Suite)," and "I Wish I Knew How It Would Feel To Be Free" written in a gospel style by Billy Taylor and Dick Dallas. There are also songs specifically created as jazz vehicles, for example Fats Waller's "The Joint Is Jumpin'," Slim Gaillard's "Flat Foot Floogie" (With the Floy Floy) or Duke Ellington's "It Don't Mean a Thing (If It Ain't Got That Swing)." These and other such songs cannot be properly sung without a jazz feeling.
Whether it is an original work, or an adaptation, a "jazz" song is defined through its use of instrumentation, improvised solos, or the general approach to performance rather than the form or structure of the composition itself. Jazz vocals often utilize elements of swing, or an idiomatic approach to syncopation, often laced with improvisation. That is how and why Louis Armstrong, Ella Fitzgerald, Betty Carter or Mark Murphy can take a straight pop song or something written for a Broadway show and transform it into a jazz song through idiomatic phrasing, syncopated placement of the beat, or improvised theme and variation. To illustrate how a traditional melody can be transformed through a "jazz" performance, see the Playlist sidebar.
The Music Division has many examples of jazz songs in the papers and special collections of Ella Fitzgerald, Anita O'Day, Carmen McRae, Billy Eckstine and others.