Fiat currency
Government-issued money such as pounds, euros or dollars, valuable by decree and trust rather than by being backed by a commodity.
Fiat currency is ordinary government money: pounds, euros, dollars, yen. The name comes from the Latin fiat, meaning let it be done, because the money has value by government declaration and collective trust rather than by being redeemable for gold or anything else.
In crypto conversation, fiat is the other side of the trade. You buy crypto with fiat and cash out back into it. The services that convert in each direction are the on-ramp (money to crypto) and the off-ramp (crypto to money), and their fees usually differ by payment method: a card purchase costs more than a bank transfer, in exchange for arriving in minutes rather than days.
A concrete example of the boundary: when you buy £100 of bitcoin with a debit card, the £100 leg moves through the ordinary banking system with its business hours, chargebacks and fraud checks, while the bitcoin leg settles on a chain that runs around the clock and does not reverse. Most of the friction in buying crypto lives at exactly that seam.
Stablecoins sit deliberately on the fence: crypto tokens engineered to track a fiat currency, most commonly the US dollar, so that value can stay inside crypto rails while prices are denominated in familiar money.