The King of all the Dúnedain or High King of the Dúnedain was a title originally taken by Elendil. As overlord of both the North-kingdom of Arnor and the South-kingdom of Gondor, he was High King over all the Dúnedain in Middle-earth. After Elendil's death in Mordor, the title passed briefly to his son Isildur, but Isildur was lost in the Disaster of the Gladden Fields. After his death, the Two Kingdoms split apart, with Isildur's descendants ruling in the North, and those of his nephew Meneldil taking up the Kingship the South-kingdom.
For almost the entirety of the Third Age, the Two Kingdoms were disunited, and there was no single King of all the Dúnedain. The Kingdoms were Reunited after the downfall of Sauron, when Aragorn II, the distant Heir of Isildur, was crowned as the third King of all the Dúnedain.