Possible that you need to look up the definition, you obviously don't know it. I'll make it easy for you though, a fiat currency is one that is not backed up by specie (look it up), gold for example. Of course both gold and BTC are "worth" what people are willing to pay for them, their value is a function of human belief. Now gold HAS a couple of practical uses, which means that it HAS a value based in measurable reality. What would you say the practical use of BTC is? BTC promoters have realized how kludgy it is as a currency and have started calling it a "store of wealth", but as a store it depends entirely on what people think about it, When people want to buy something with their BTC they convert it to a usable currency, like the Euro, or the Dollar, or based on this article, the Nano, all of which have utility to make purchases;)