The dictionary definition of a constitutional monarchy is "a form of government in which a monarch acts as head of state within the parameters of a constitution."
In essence, it is a system of government with a monarchy, but the monarch does not have absolute power.
The power of a monarch in a constitutional monarchy can vary dramatically. For example, both Jordan and the UK are called constitutional monarchies. The UK is a liberal democracy where the Queen has little control over executive and legislative power. In Jordan however, most of the executive power is exercised by the King.
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