Money that (a) derives its value entirely from the mandate of the government, and (b) cannot be freely traded. Fiat money is not the same thing as floating currency, because if a floating currency is intrinsically worthless then its lack of worth will be reflected in the forex markets. Fiat money, on the other hand, does not require a disciplined monetary of fiscal policy on the part of the issuing authorities; exchange rates are fixed by decree, which means the state also controls supplies of hard (foreign) currency.
Money that is valueless, or that is backed by nothing, or by such a small amount of any tangible compared to its perceived value that its tangible value is not worth noting
