><< Here's what I think of a cadenza:
No doubt Bradley will give us the definitive word, but my music master at school said that the cadenza was a solo section in a concerto usually sketched in by the composer (basic chord sequences, key changes and the like) so that the performer had something to go by. You didn't have to vary what the composer had sketched in but it was usually better to do so because the sketched music was pretty boring (although I think some composers wrote pretty good cadenzas and expected performers to stick rigidly to them).