In both the North- and South-kingdoms of the Dúnedain, there had been no King for many centuries. At the time of Aragorn's coronation, there had been no King of Gondor for 969 years and no King of a united Arnor for 2,158 years. The last King to rule both realms jointly had been Aragorn's distant ancestor Isildur, though his Kingship was cut short at the Disaster of the Gladden Fields before he could truly take up the rule of his twin realms.
From that time, when the Third Age was just two years old, the West had had no uniting King. At the coronation of Aragorn, two years from the Age's end, Arnor and Gondor were at last united once again, and Aragorn's rule of the West is therefore referred to as the Reunited Kingdom.