A defined exchange rate is one that is fixed by law. One example would be the Bretton Woods Accord which set the exchange rates for all participating currencies in terms of dollars, then set a fixed exchange rate for dollars in terms of gold (albeit redeemable only by central banks).
So you are saying you can’t give a current example of a “defined” (by your definition) exchange rate. But for some reason I am supposed to provide one to you for a virtual currency. Do you see the double standard?