The concept is from the days of the classical gold standard.
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So as long as no government has declared that bitcoin is the money that should be used in the land, bitcoin is not fiat.
That is an odd way to redefine fiat. Under that definition, a commodity-backed currency becomes fiat if the government proclaims it so.
That's right. I am also aware that the word is used nowadays to describe paper money.
Fiat paper money and bitcoin both have no value for direct use, that is probably the connection, but bitcoin is not fiat.