A currency that is declared as legal tender with support from a government legislation.
The Long Explanation
It is a currency that gets its value from the government that issues it rather than a physical commodity.
Fiat money is a currency that doesn’t have any innate value as it is not backed by a physical good or commodity, but is still accepted as a valid payment method in many places.
There is now growing support for the use of fiat currency as an alternative payment method for the purchase of NFTs.
OpenSea, the leading NFT marketplace, has plans to give its customers the ability to use fiat currencies (ie. with credit card) to purchase NFTs.