High King of the Dúnedain
High King was a title used by the Dúnedain for their ultimate ruler. Normally, "High King" would refer to a king who ruled over other lesser kings and lords, but this is not necessarily the case with the High King of the Dúnedain. The term seems to date back to the early days of Númenor, where Tar-Meneldur the fifth King was addressed as "High King", though he had no other rulers in his service.
The more common use of the title, though, came at the end of the Second Age, with the establishment of the Realms in Exile in Middle-earth. In their earliest years, the Two Kingdoms of Arnor and Gondor fell under the ultimate rule of one man, Elendil, who dwelt in Arnor as the High King. The South-kingdom of Gondor was ruled jointly by his sons, but only under his suzerainty.
With Elendil's loss in the Siege of Barad-dûr, his elder son Isildur inherited the High Kingship. Isildur ruled for only two years, though, before he too was lost in the Disaster of the Gladden Fields. After his death, historical events conspired to separate the Two Kingdoms, each of which took Kings of their own. The High Kingship should in principle have fallen on Isildur's heir Valandil, but in practice Valandil became King of Arnor only, and had no power over the South-kingdom.
More than three thousand years were to pass before Aragorn reunited Elendil's realms. In doing so, as the direct descendant of Isildur through many generations, he also resurrected the High Kingship.