King is the title given to a male monarch in a variety of contexts. The female equivalent is queen regnant (while the title of queen on its own usually refers to the consort of a king).
The English term 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 . The English term "king" 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 ríg, but see Gothic reiks and, e.g., modern German Reich and modern Dutch rijk). It is a derivation from the term *kunjom "kin" (Old English ) by the suffix. The literal meaning is that of a "scion of the [noble] kin", or perhaps "son or descendant of one of noble birth" (OED).
English queen translates Latin regina; it is from Old English cwen "queen, noble woman, wife" from the PIE word for "woman" (*gwen-). The Germanic term for "wife" appears to have been specialized to "wife of a king"; in Old Norse, the cognate kvan still mostly refers to a wife generally. Scandinavian drottning, dronning is a feminine derivation from "lord".
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.