FIAT: (noun) /ˈfiː.æt/
: an authoritative or arbitrary order : DECREE
: an authoritative determination : DICTATE
: a command or act of will that creates something without or as if without further effort

~ merriam-webster.com/dictionary

FIAT: from Latin fieri «to be made, come into being»

Fiat is money that has value only because the government says (decrees) that it does.

Therefore people (have to) believe it does.

Even if they don't believe fiat has value, by law they are forced to use it and to accept it as payment for goods and services.

Fiat money is printed/created out of thin air.

These days about 5% of all dollars are printed as cash.

The other 95% is digitally created by banks entering numbers into a computer when they issue loans.

It costs only a few cents for the Bureau of Engraving and Printing to produce a $100 bill…

~ American Economist, Barry Eichengreen