the definition of jazz is much broader than many people think. wikipedia has a good article on Jazz

http://en.wikipedia.org/wiki/Jazz

in many cases a prefix is added to the word Jazz to add to the description eg smooth jazz, trad jazz, acid jazz etc etc and jazz can mean many different things to many people

What is Jazz?
Jazz is a kind of music in which improvisation is typically an important part. In most jazz performances, players play solos which they make up on the spot, which requires considerable skill.

There is tremendous variety in jazz, but most jazz is very rhythmic, has a forward momentum called "swing," and uses "bent" or "blue" notes. You can often hear "call--and--response" patterns in jazz, in which one instrument, voice, or part of the band answers another.

Jazz can express many different emotions, from pain to sheer joy. In jazz, you may hear the sounds of freedom-for the music has been a powerful voice for people suffering unfair treatment because of the color of the skin, or because they lived in a country run by a cruel dictator.

Jazz is about making something familiar--a familiar song--into something fresh. And about making something shared--a tune that everyone knows--into something personal.

Jazz developed a series of different styles including traditional jazz, swing, bebop, cool jazz, and jazz rock, among others.

Over the last century  jazz  has  spread from the United States to many parts of the world, and today jazz musicians--and jazz festivals--can be found in dozens of countries.

Those are just some of the reasons that jazz is a great art form, and why some people consider it "America's classical music." 
Smithsonian Copyright National Museum of American History

Some interesting thoughts on What is Jazz. I know one presenter at 94.1fm who had the opinion that Jazz is indefinable - almost a case of "what you want it to be". Perhaps the more difficult question is "What isnt Jazz?"

Jazz of late has morphed into something called World Music. If you want to see how jazz has degenerated into a disconnect from its origins have a look at the Montreaux Festival of late.
I am no purist preferring to think of jazz as basically something improvised over a predetermined harmonic progression. Maybe you should add a pinch of rhythm and a teaspoon of syncopation if you're into "swing"
What I love about it is the endless seeking of something new and not just for a monetary reward.
I am a working musician and have been for more years than I care to remember although semi-retired. You never retire from jazz or music making of any genre for that matter.If money is your motive forget about it.
Most jazz musicians think firstly in terms of what they can contribute, secondly for respect from their colleagues although not necessarily in that order.
The jazz life never ends. Even after death people will remember the likes of Ellington, Armstrong and Diz.

I like that jazz is 'freestyle' music. In which the musicians respect each other's contribution. And speaking of that I'm looking for John Foreman's original jazz album.  Anyone have it?