Yes, but you can't arbitrarily redefine words. People need to understand each other, and that's why there are words with agreed meaning.
And the agreed meaning of fiat currency is a currency which value is regulated by a government.
That definition would be wrong. The value of a currency is determined by what other people are willing to trade for it. The government may try to regulate the value by pegging a currency to another government fiat currency, forbidding non-government exchanges, or fixing prices on goods, but inevitable black markets and black market currency exchanges will give you the true value.