According to Wikipedia, fiat currency or fiat money is the following:
Fiat money is a currency without intrinsic value established as money by government regulation or law.
In general, fiat currencies are any currencies that are issued by a government by regulation or law. Examples are the United States Dollar (USD), the Eurozone euro (EUR), the United Kingdom’s pound sterling (GBP), the Japanese Yen (JPY) and the Chinese Yuan (CNY).
Fiat currencies can be deposited to or withdrawn from your Kraken account via bank transfers from a number of countries around the world.