[epscylonb]

The definition and use of the word fiat is interesting, in Latin it means "let it be done".

You could say that the creator of Bitcoin decreed that it had value (I'm not sure I would agree), but it's still different from government backed money because nobody is forced to use Bitcoin (to pay taxes for example).

It certainly seems that within the Bitcoin community the word fiat now means government backed money, I suspect if Bitcoin becomes more successful it may end up redefining the word.

[thaumasiotes]

So, in the historic situation in which a farmer's business is all conducted in copper (or in kind) and taxes are assessed in silver, would you consider silver a fiat currency?

A fiat currency is fiat because it gives you no claim to anything. A pound of silver gives you a claim to a pound of silver. A US dollar used to give you a claim to a certain weight of gold. Today the price of gold is uncontrolled.

(Side note: owing to the special nature of facio/fio, using "fiat" doesn't necessarily require an agent. The verb is indeed the passive form of "facio" [do/make], but it's also the standard way to express [happen/occur].)