A monarchy is a form of government in which a natural person, the monarch, is head of state until death or abdication. The governing power of the monarch may vary from purely symbolic or partial (crowned republic), to restricted (constitutional monarchy), to fully autocratic (absolute monarchy), combining heads of state, government, legislature and judiciary.
There are elective, self-proclaimed or as in most cases, hereditary monarchies. For many such cases aristocracy, though not inherent to monarchies, serves as the pool of persons to draw the monarch from and fill the constituting institutions (e.g. diet and court), giving many monarchies oligarchic elements.
A monarchy can be a polity in personal union, vassalage, federation or unitary. Its authorities are proclaimed and recognized through the different seats, insignia and titles that a monarch can occupy and be invested with. For example, monarchs can carry titles such as king, queen, emperor, khan, caliph, tsar, or sultan, and can be bound to territories or peoples, as in Emperor of Japan or King of Belgians.
Strictly speaking has the republic become the opposing form of government to a monarchy, though there have been infringements of this core principle of republics, legitimating lifetime or hereditary rule. Presidents are often the republican counterpiece to monarchs as heads of state.
Monarchy was the most common form of government until the 20th century. Forty-five sovereign nations in the world have monarchs acting as heads of state, sixteen of which are Commonwealth realms that recognise Queen Elizabeth II as their head of state. Most modern monarchs are constitutional monarchs, who retain a unique legal and ceremonial role, but exercise limited or no political power under the nation's constitution. In some nations, however, such as Brunei, Morocco, Oman, Qatar, Saudi Arabia, and Eswatini, the hereditary monarch has more political influence than any other single source of authority in the nation, either by tradition or by a constitutional mandate.
Although most contemporary monarchies are nations, historically, monarchic polities are not to be understood through the logic of the nation state or even a classic territorial state. A nation or constitution is not necessary in a monarchy since a person, the monarch, binds the separate territories and political legitimacy (e.g. in personal union) together.