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 (crowned republic), to restricted (constitutional monarchy), to fully autocratic (absolute monarchy), combining executive, legislative and judicial power.
In most cases, monarchies are hereditary, but there are also elective and self-proclaimed monarchies. Aristocracy, though not inherent to monarchies, often 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 that is a federation, a unitary state, in personal union or in vassalage. 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 (e.g., the Emperor of Japan) or peoples (e.g., the King of the Belgians).
The main alternative to monarchical form of government in modern times is republican, though there have been infringements of this core principle of republics with lifetime or even hereditary presidents. Republics’ heads of state are often styled "President" or a variant thereof.
Monarchy was the most common form of government until the 20th century. Forty-five sovereign nations in the world have a monarch as head of state, including sixteen Commonwealth realms that each have Queen Elizabeth II (in separate capacities). 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 United Kingdom, 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.
Historically, monarchies pre-dated such polities as nation states and even territorial states. 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.