High King of the Dunedain

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.
In Gil-galad's letter, Tar-Meneldur the fifth King of Númenor, was referred to as "High King", though he had no other rulers in his service.
== Realms in Exile ==
The more common use of the title, though, came in the later Second Age, with the establishment of the Realms in Exile in Middle-earth. In their earliest years, the Two Kingdoms of Arnor and Gondor were formed 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 death against Sauron 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 before he could truly take up the rule of his twin realms.
=== Decline ===
After Isildur's 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 the Heir of Isildur Valandil, but in practice Valandil became King of Arnor only, and had no power over the South-kingdom.
In the mid-Third Age Gondor was left kingless; Arvedui, as Isildur's heir, attempted to invoke the Law of Succession in Númenor to claim the throne of Gondor, thus becoming High King; the Council of Gondor voted against his claim. Shortly after, both realms would lose their kingship.
== Reign of Elessar ==
Before the War of the Ring, there had been no King of Gondor for 969 years, and no King of a united Arnor for 2,158 years. After Sauron's passing Aragorn the direct descendant and Heir of Isildur, as well as Anárion, through many generations, ascended to the throne of Gondor.
Aragorn took the title of King of the West also King of Gondor and Arnor and Lord of the Westlands (S. Hîr i Mbair Annui) signifying his Kingship over all the Free peoples of the Westlands, from the north to the south. His rule of the West is therefore referred to as the Reunited Kingdom.
Under his crown and protection were other kingdoms, like the Kingdom of Dale and the Kingdom Under the Mountain.