Legal tender, often in the form of paper currency.
Fiat (money) is the currency form that the government of a region deems and approves to be legal tender. In other words, fiat is the means and medium of exchange that has been approved by constituted authority. Thus, the value of fiat money depends on the government rather than on public sentiment. Moreover, this form of money is almost never reliant on a precious metal like gold, so it is generally only representative of exchange value and has no inherent value in itself.
Fiat is usually in paper form, with government emblems inscribed on it to show its authenticity.