A fiat is an official order from someone in power who does not stop to ask what other people want. A ruler, a court, a government, sometimes a boss just says it, and it happens. You will mostly meet the set phrase “by fiat,” which means “because they ordered it, not because people agreed.” It is a formal, slightly stiff word; friends do not use it about who washes the dishes. Though actually, people stretch it. “Fiat money” is ordinary cash that has value because the state says so, not because it is gold in your pocket — that is the same idea of “because we declared it.” As a verb, to fiat something through (to push it through by order) exists, but it is rare. The car company Fiat uses the same letters; that name is not this word.