Jazz has more than one definition, and jazz has different definitions to different people. It also can mean different things to the same people at different times. Some think about a quality of swing when they define jazz - a sound that evokes a feeling of wanting to tap your feet or clap your hands. One quality of jazz common to all definitions is improvisation - to ad lib, fake, ride or jam. This is when a musician composes and performs simultaneously.
Some musicians use jazz as a synonym for improvisation.
Some audiences will recognize improvisation as being when a soloist in a group stands up, or when musicians stand up after playing a familiar melody, launch into unfamiliar sounds. Creating melodies spontaneously is a joyful feeling, as natural as wanting to sing on the spur of the moment about something thrilling - a thought, a feeling, a vision, or a sound. For a performer to transfer that emotional feeling to others, he or she must have technical expertise. Creating spontaneously takes skill. The quality of improvisation depends on the performer's depth of thought.