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. [1]
Fiat money is an intrinsically worthless object, such as paper money, that is deemed to be money by law.
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I will say that when Japan agreed to accept payment of taxes, with Bitcoin, they deemed it to be money.
Probably I could find examples in the U.S. government prosecutions of criminals who used Bitcoin, that had government lawyers advancing arguments that treated Bitcoin as money, but, I don't have them to hand.
I'm really interested in figuring out if this is true. What source is saying this? From what I can tell some taxes might apply to Bitcoin in Japan but in no way can you actually "pay" taxes in Bitcoin (you can obviously convert it to Yen first though).