The term “fiat” is often associated with irredeemable-paper or electronic currency, but existing only in paper or electronic form is not the defining characteristic of fiat currency. In fact, paper or electronic currency is not necessarily “fiat” and hard commodity currency can be “fiat”.
Regardless of the form it takes, fiat currency is simply currency by government decree. If the government dictates that a certain ‘thing’ is money and must be accepted in payment for goods, services and debts, then that ‘thing’ is a fiat currency.
Obviously, all of today’s national currencies are fiat currencies. Not so obviously, gold was a fiat currency during the Gold Standard era. It could be claimed — without any argument from me — that during the Gold Standard era gold would have been the most widely used currency without the government making it so, but this is beside the point. In the situation where the government has commanded that gold is money, gold is a fiat currency.
Also not so obviously considering what has been written on the topic in other places, Bitcoin is not a fiat currency. If anything it is the opposite of a fiat currency, because it was created by the private sector and is not supported in any way by the government. This doesn’t mean that Bitcoin is a good currency, as there is a lot more to being a good currency than being outside the direct control of government.
Summing up, people should be careful when applying the word “fiat” to currency/money. The word is routinely used to mean irredeemable or non-physical, but that’s not what it actually means.