fiat - an arbitrary decree or pronouncement, especially by a person or group of persons having absolute authority to enforce it.

The amount fiat currency can be doubled overnight by the authority over the fiat currency. Stealing, in effect, from all of the people holding that currency. Non-fiat, like gold or bitcoin (at least this is what the true believers believe), have a high amount of work that needs to be done to just get a little bit more of it. No grabbing half the currency overnight.