Fiat currency is currency because it is declared so 'by fiat.' Even if gold were a currency, it would not be fiat, because gold has a value independent of its value as a currency.
Yup. I should point out that
technically gold can be a fiat currency if we declare "by fiat" that an ounce of gold is worth a hundred zillion trillion dollars (rather like a penny is fiat currency despite the fact that the zinc it contains is worth a hundredth of a cent or something like that. I guess technically even currency has some intrinsic value if you need to light a fire badly enough or something.)
But the key property of fiat money from an economic standpoint is that no sensible person would use it as currency if it weren't backed up by a collective hallucination taking the form of law/policy. If the Fed suddenly repudiated FRN's and would not longer accept them, you'd see the value plummet like, er, um,.... a thing that plummets very quickly.
And that's actually a good thing to sensible economists, because it means, first, that currency tends to circulate. We had a problem in the States twenty-five or so years back when copper prices started to rise, and the then-copper pennies were starting to be worth more for their copper value than their face value. This meant that some people were taking pennies out of circulation -- in some cases, thousands or hundreds of thousands of pennies. (I blush to admit that my mother was among those people; I think we still have something like 50,000 rolled copper pennies lying around our basement against the rise in copper prices.) Taking money out of circulation like that, especially on a large scale, does nasty things to the economy.
So the Fed switched to zinc, which was much cheaper, and today pennies circulate again.
Imagine what would transpire if this had happened to all forms of currency, top to bottom, so it wasn't just incidentals like pennies that were being taken out of circulation.
The other advantage, and this is even more important, is that by using nearly-valueless fiat money, the government can easily and cheaply control the amount of money in circulation.
Of course, Tippit believes counterfactually that this is a bad thing. He apparently likes the idea of uncontrolled liquidity crises from time to time, and occasional unexpected bouts of runaway inflation when someone opens a new mine or finds a new way to extract gold from seawater. He likes the idea that his supply of gold will always be worth exactly $1 even if he has no idea whether a loaf of bread will cost a penny or a hundred dollars.
Sensible people think the other way; they'd rather be able to predict the price of bread than the price of gold.