I agree, the random notes are improvisation. But improvisation can also be done with non-random-sounding notes: Mozart's variations on "Twinkle, Twinkle, Litt

I agree, the random notes are improvisation. But improvisation can also be done with non-random-sounding notes: Mozart's variations on "Twinkle, Twinkle, Little Star" are very impressive. The difference being that with Mozart you can still hear a melody. You'd have to be really good to do this on the fly in a solo, though, which is probably why a lot of the great jazz bands practiced beforehand (it wasn't strictly improv)