The correct term for Bitcoin currency
The general term in widespread use today is cryptocurrency. This refers to a currency designed upon cryptographic principles (as Bitcoin is).

The term "fiat money" refers to the usual money issued by governments. The government declares "this is money" (even though the paper on which it is printed does not have value itself), and people use it as such. Wiktionary defines fiat as:


  
  An authoritative command or order to do something; an effectual decree.