Fiat is government-issued money, like dollars or euros, whose value rests on legal tender status and central bank policy rather than a commodity such as gold.
Fiat and crypto are different systems, but they interact: prices are quoted in fiat, and fiat on-ramps and off-ramps are how users buy and sell crypto.
Note: Fiat isn’t an acronym. It comes from Latin “fiat,” meaning “let it be done” or “by decree.