Let's see what the Internet says about it:
Fiat money is currency which derives its value from government regulation or law.
Completely wrong. Governments can regulate commodities and tell you how much they are worth, but it doesn't make it a fiat currency.
Currency that a government has declared to be legal tender, but is not backed by a physical commodity.
Legal tender, meaning that you can pay a government debit with it. A generally accepted definition of fiat currency, in that it is not backed by a commodity.
money (as paper currency) not convertible into coin or specie of equivalent value
That makes no sense. Misquote?
Paper money or coins of little or no intrinsic value in themselves and not convertible into gold or silver, but made legal tender by fiat (order) of the government.
Sounds close.
To be fair, from my quick search, there's one site (Merriam Webster) that mentions nothing about the government; but others do. So, you can't just say That's not the definition of fiat when most people disagree with you.
Here is the rub: If you classify currency as EITHER fiat OR a commodity or commodity-backed currency (USD before Nixon), then I think bitcoin is a fiat, with the "government" being the consensus of miners regulating the issuance of bitcoin. You just can't go back to the miner and ask for some electricity and hardware in exchange for the bitcoin they mined that you currently hold.
The term "government" in relation to fiat currency is vague. US dollars are issued by a private bank AKA the Federal Reserve that is not really controlled by the US government. Euros are not issued by one government, but rather a cartel of governments.