A constitutional monarchy is a system of government in which a monarch shares power with a constitutionally organized government. The monarch may act as the effective head of state or serve mainly as a ceremonial leader, while the constitution assigns the rest of the government’s authority to the legislature and judiciary.
Britain developed into a constitutional monarchy under the Whigs; other examples include Belgium, Cambodia, Jordan, the Netherlands, Norway, Spain, Sweden, and Thailand.
Source: Encyclopaedia Britannica — constitutional monarchy.