King

King is the title given to a male monarch. A king is an absolute monarch if he holds the powers of government without control, or the entire sovereignty over a nation; he is a limited monarch if his power is restrained by fixed laws. Kings are hereditary sovereigns when they hold the powers of government by right of birth or inheritance, and elective when raised to the throne by choice.
The term king may also refer to a king consort, a title that is sometimes given to the husband of a queen regnant, but the title of prince consort is more common.
The English term king is derived from the Anglo-Saxon cyning, which in turn is derived from the Common Germanic *kuningaz. The Common Germanic term was borrowed into Estonian and Finnish at an early time, surviving in these languages as kuningas. It is a derivation from the term *kunjom "kin" (Old English cynn) by the -inga- suffix. The literal meaning is that of a "scion of the [noble] kin", or perhaps "son or descendant of one of noble birth" (OED).
The English term translates, and is considered equivalent to, Latin rēx and its equivalents in the various European languages. The Germanic term is notably different from the word for "King" in other Indo-European languages (*rēks "ruler"; Latin rēx, Sanskrit rājan and Irish rí; however, see Gothic reiks and, e.g., modern German Reich and modern Dutch rijk).
The English word is of Germanic origin, and historically refers to Germanic kingship, in the pre-Christian period a type of tribal kingship. The monarchies of Europe in the Christian Middle Ages derived their claim from Christianisation and the divine right of kings, partly influenced by the notion of sacral kingship inherited from Germanic antiquity.
The Early Middle Ages begin with a fragmentation of the former Western Roman Empire into barbarian kingdoms. In Western Europe, the kingdom of the Franks developed into the Carolingian Empire by the 8th century, and the kingdoms of Anglo-Saxon England were unified into the kingdom of England by the 10th century.
With the breakup of the Carolingian Empire in the 9th century, the system of feudalism places kings at the head of a pyramid of relationships between liege lords and vassals, dependent on the regional rule of barons, and the intermediate positions of counts (or earls) and dukes. The core of European feudal manorialism in the High Middle Ages were the territories of the former Carolingian Empire, i.e. the kingdom of France and the Holy Roman Empire (centered on the nominal kingdoms of Germany and Italy).
In the course of the European Middle Ages, the European kingdoms underwent a general trend of centralisation of power, so that by the Late Middle Ages there were a number of large and powerful kingdoms in Europe, which would develop into the great powers of Europe in the Early Modern period.
By the end of the Middle Ages, the kings of these kingdoms would start to place arches with an orb and cross on top as an Imperial crown, which only the Holy Roman Emperor had had before. This symbolized them holding the imperium and being emperors in their own realm not subject even theoretically anymore to the Holy Roman Emperor.
Currently (as of 2023), eighteen kings are recognized as the heads of state of sovereign states (i.e. English king is used as official translation of the respective native titles held by the monarchs).
Most of these are heads of state of constitutional monarchies; kings ruling over absolute monarchies are the King of Saudi Arabia, the King of Bahrain and the King of Eswatini.