I am not sure it can be defined in a definitive sense.
A musicologist would talk about the historical roots. Southern black music... spirituals, blues, gospel etc... And that would be very informative and very true. The great French Jazz violinist Stephane Grappelli once said, 'I never forget, it's a black art form'. He played it well, and made his contributions via his performances, but he never tried to define it or re-define it. This is the heart of the problem. Not so much, WHAT is Jazz, but, WHO should define Jazz. This problem exists in many genres of music.
The best answer for me is, to paraphrase the Supreme Court Justice's views about obscenity, I can't define Jazz, but I know it when I hear it. So we all live in different musical universes, and in my universe, the definition of Jazz is stored on that hard drive between my ears. Put there bit by bit during the years of my musical experiences.
One contributing factor in all this confusion concerning genre, is the idea, prevalent in this country, that a person can reach any goal, with hard work. This is a good positive idea in a general sense, but not absolutely true. When it comes to the arts, talent is the deciding factor. Talent trumps everything else. Desire and hard work are helpful, but talent is vital element. Some refuse to let a lack of talent stop them from reaching their goals.
The result is, we have guys from London calling themselves bluesmen. People from europe playing 'euro' Jazz. They make a lot of money and some of it sounds ok, but it ain't the real deal. Stephane never called his music 'euro Jazz'. hmmmmmm. I think the low point in my musical experience was listening to a German Country and Western band in Franfurt. I still cringe!!
Orpheus10: great question