Who has the authority to issue fiat currency?

The Government does.

The word "fiat" refers to a decree or an order issued by an authority or government with the power to do so. In the context of fiat currencies, the term "fiat" refers to the fact that these currencies are not backed by a commodity such as gold or silver but are backed by the government's guarantee of their value.

Fiat currencies are legal tenders issued by governments and used as a medium of exchange for goods and services. They derive their value from the government's backing and the faith and confidence that people have in the stability of the government and its economy. The value of fiat currency can be affected by various factors, including inflation, interest rates, and government policy.

The term "fiat" is often used to distinguish these currencies from other forms of money that have intrinsic value, such as gold or silver coins.

In the fiat system, you have to believe in the full faith and credit of the government that issues the currency.