Hesperis said :Improvisiong is just creating a flow of tension and release over the duration of the song.

I agree, but would equate this definition more akin to jazz, but the concept is relative. I believe it was Tony Quarrington who had one of the best definitions of "jazz music" I've ever heard. Jazz is "creating problems, and then solving them."

This can be applied to all forms of improvisation methinks. It's just a question of the simplicity or the complexity of the created problem.