Jazz is the MOST significant form of music in American history, and it has become an international phenomenon as it continues to evolve. It is somewhat elusive to define because it encompasses so much.
When they talk about Jazz, they talk about "Improvisation." Improvisation is the creative ability to make the song your own. In order to depart from the original sheet music, one has to know and understand the song beyond the average Joe. To break the rules, one has to know and understand the rules. Jazz musicians, and singers, are the best in the business.
Dr. Billy Taylor on Improvisation - very interesting!
What is unique about jazz is that everyone has his or own definition of what it is and who performs it. Remember when MY HEART was riding the charts last year? It most popular designation (on Amazon.com) was as VOCAL JAZZ. Oh well....