Русско-англо-русский юридический словарь

The head of state is the highest-ranking constitutional position[note 1] in a sovereign state. The head of state is vested with powers to act as the chief public representative of that state. Heads of state in most countries are natural persons holding an office. In a monarchy the reigning monarch is the head of state, though the title may not be king or queen. In a republic the head of state is usually titled president, but may have other titles such as chairman. (Wikipedia)

Later Polybius in his analysis of the Roman Constitution used the concept of aristocracy to describe his conception of a republic as a mixed form of government, along with democracy and monarchy in their conception from then, as a system of checks and balances, where each element checks the excesses of the other. (Wikipedia)

A monarchy is a form of government in which a person, the monarch, is head of state for life or until abdication. The political legitimacy and authority of the monarch may vary from purely symbolic (crowned republic), to restricted (constitutional monarchy), to fully autocratic (absolute monarchy), and can expand across the domains of the executive, legislative and judicial. A monarchy can be a polity through unity, personal union, vassalage or federation, and monarchs can carry various titles such as king, queen, emperor, Raja, khan, caliph, tsar, sultan, or shah. (Wikipedia)

A constitutional monarchy is a form of monarchy in which the sovereign exercises authority in accordance with a written or unwritten constitution. (Wikipedia)