Fiat is the national currency. All live in different countries and all have different currencies. Fiat is a universal concept that is identical to the euro, dollar, other currencies. Since the market is not stable and bitcoin price is also floating, some take the national currency rather than the dollar as the starting point. This term is used in the context of the withdrawal of investment in real life, as within the cryptocurrency market, money moves through bitcoin, ether or Neo, etc.