Have yet to see a crypto-currency that been made legal tender by a government decree. (though, admittedly, it's possible that there's one backed by a nation-state I haven't heard about)
fiat isn’t synonymous for government: it just means there’s no value except what people agree upon. That makes Bitcoin a pure fiat currency which is especially volatile because there’s no inertia from demands that, say, taxes be paid in it or that civil servants and government contracts use it.
Fiat (in the current context) literally means "by government decree".
I believe what you mean is that modern currencies (and bitcoin) aren't backed by anything other than pure belief they hold value as opposed to their traditional backing by specie or bullion.
> Fiat money is money that some authority, generally a government, has ordered to be accepted as a medium of exchange.
The core idea is that "some authority" doesn't assign the value to the money but requires it to be accepted under penalty of the law.
Usually, historically speaking, when a currency is separated from its underlying commodity by fiat it retains its value due to the fact that people have associated it with the value of the commodity -- up until the point "some authority" fires up the printing presses and the historic link is broken.
Anyhoo, not that I'm disagreeing with you on principal it's just that "fiat currency" has a specific meaning that "some authority" has spent a lot of energy trying to disassociate it from.
Have yet to see a crypto-currency that been made legal tender by a government decree. (though, admittedly, it's possible that there's one backed by a nation-state I haven't heard about)