The Reunited Kingdom was the restored kingdom of Arnor and Gondor, the twin kingdoms founded at the end of the Second Age by Elendil and his sons Isildur and Anárion. In the beginning, Elendil was High King of both realms, but the two were divided after the deaths of his sons. Over 3,000 years later Aragorn Elessar reunited the kingdoms and ruled as High King of the Reunited Kingdom.
The fate of the kingdoms, especially that of Gondor, would come to dominate the history of the Third Age.
At the end of the Second Age, Elendil and his sons established two great kingdoms in Middle-earth: Arnor in the north and Gondor in the south. These Kingdoms of the Dúnedain were united under a single High King, Elendil himself, who ruled the North-kingdom while his sons reigned jointly over the South-kingdom of Gondor.
In the first years of the Third Age, this union failed. After the loss of Isildur at the Disaster of the Gladden Fields, his nephew Meneldil took up the kingship of Gondor, and that country remained independent from the North-kingdom through most of the Third Age.
In the north, the realm of Arnor fell into troubled times. It broke into three separate kingdoms, and in time these too were lost, so that the Dúnedain of the North-kingdom of Elendil were reduced to a wandering people led by a Chieftain. Nonetheless, they were able to maintain Isildur's line in unbroken descent.
There were two attempts to reunite the kingdoms. The first of these occurred in III 1944, when Arvedui of Arthedain claimed the High Kingship of the Two Kingdoms. His claim was rejected by Steward Pelendur and the Council of Gondor, who elected to maintain their independence.
More than a thousand years later, after the War of the Ring, Arvedui's direct descendant Aragorn came forward to make the same claim. This time, the people of Gondor accepted a High King, and the Two Kingdoms were reunited at last.
Like his ancestor Elendil before him, Aragorn took up his rule from the North-kingdom, but he travelled throughout his wide lands. The Shire was an exception to this, and though it lay within the Reunited Kingdom, Aragorn made a law that Men should not enter it, a law that he observed himself. Though his seat was in the north, Minas Tirith and the South-kingdom remained important, to the extent that he travelled there at the end of his life, and his tomb was among the Houses of the Dead beneath Mount Mindolluin. After Aragorn's death, his son Eldarion took up the High Kingship, and the Reunited Kingdom endured for many years under the new King and his descendants.