John Lennon helped to shape rock and roll for a generation and forever changed how a generation saw music. He was an icon and a symbol of his generation. When Lennon was shot it symbolized the end of an era for many, and the sudden loss of his influence on the world was shocking. Furthermore, Lennon inspired numerous musicians who came after him, and it is doubtless that his death had an affect on their music as they dealt with this loss.
John Lennon, as a member of the Beatles, help to define what is considered rock and roll music. He was one of the faces of the Baby Boomers that lead the "end war" peace movement. His death marked the end of one era and the beginning of another for new generation of listeners.
The death of John Lennon was a great loss to the musical word but to call it a defining moment for the music world is a little much. Musicians were shot and killed before Lennon but his was more public due to his Beatles fame. There was music before Lennon and there was music after Lennon.
Musicians come and go, whether by retirement, they disappear as one-hit wonders, or death. While Lennon was a music icon, he is not the first, nor will he be the last, rocker to die. Music is bigger than the individual creating it. Music lives on even when the artist does not.