A constitutional monarchy is a form of monarchical government established under a constitutional system which acknowledges an elected or hereditary monarch as head of state. Modern constitutional monarchies usually implement the concept of trias politica or "separation of powers", where the monarch either is the head of the executive branch or simply has a ceremonial role. Where a monarch holds absolute power, it is known as an absolute monarchy. The process of government and law within an absolute monarchy can be very different from that in a constitutional monarchy.
In representative democracies that are constitutional monarchies, like the United Kingdom, the monarch may be regarded as the head of state but the prime minister, whose power derives directly or indirectly from elections, is head of government.
Although current constitutional monarchies are mostly representative democracies (called constitutional democratic monarchies), this has not always historically been the case. There have been monarchies which have coexisted with constitutions which were fascist (or quasi-fascist), as was the case in Italy, Japan and Spain, or with military dictatorships, as is currently the case in Thailand.