Jazz almost defies definition.
There are many types and styles of jazz and as we get further and further away from the beginnings of jazz, many elements fuse together to add to the confusion. Jazz was more of a separate entity years ago, but now we have jazz-rock, latin jazz, acid jazz, fusion and several others. How jazz is defined depends on whom you talk to. The dictionary itself has several definitions ranging from those with very strict confines, to those that are more inclusive and general in nature. It developed around the turn of the century.