Which type of government is one in which a single leader has the power

Monarchy - a government in which the supreme power is lodged in the hands of a monarch who reigns over a state or territory, usually for life and by hereditary right; the monarch may be either a sole absolute ruler or a sovereign - such as a king, queen or prince - with constitutionally limited authority.

Explanation:

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A monarchy is a government in which a monarch has supreme power over a state or territory. Usually this power is given for life by a hereditary right.

The monarch may also have absolute power (absolute monarchy) or their power may be constitutionally-limited (constitutional monarchy).