Definition: rooted in the strong vocal tradition of the blues, vocal jazz evolved out of a desire to improvise vocally the way instrumentalists did.
Jazz singers take liberties even when singing the composed portion of jazz compositions (see “the head”). They often embellish the melody with melodic ornamentation and with exaggerated inflection. When they improvise, they sing streams of nonsense syllables that vaguely mimic what an improvising saxophonist or trumpeter might sound like. This technique is known as scat singing.