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From latin regnum, reign is any territory whose residents are subject to a King. It is a State governed by a monarchy, which is a form of Government in which the supreme position is individual (for one person), to life and, generally, hereditary.
Examples: "her Majesty has defended his reign with vigor and heroism", 'our reign extends from the sea in the South to the mountains of la Paz', "In the middle ages, the monarch ordered the construction of more than twenty fortresses to defend his reign of barbaric attacks."
In a (meaning also 'Kingdom') reign, the power of the monarch may be absolute (the King is not required to render accounts which in its decisions and has no limits when ordering) or limited (the function of the King is governed by the Constitution).
At present, most of the reigns are governed by constitutional monarchies. The King, in these cases, follows a protocol function, acting as a symbol of national unity. Sometimes, the King may even act as an arbitrator with the various political parties.
The concept of reign is also used figuratively to refer to the framework of an activity. It is understood that the reign of a person is the domain where it is able to exercise some control: «There person like Messi in the reign of football», "Biochemistry is my rule: I feel quite uncomfortable.
For biology, a reign is one of the major subdivisions which allow to distribute human beings natural based on their common characteristics. Thus, we can speak of animal, vegetable and mineral Kingdom.