Regent of Gondor
Regent, a translation of the Elvish term Karma-kundo which means "Guardian of the Helm", that was applied to those granted executive authority in place of a legitimate reigning King of Gondor. In all the history of the South-kingdom, only one Regent of this kind was ever appointed, Minalcar son of Calmacil.
Minalcar was the grandson of Atanatar II Alcarin, and already in the closing years of his grandfather's rule, he had become a forceful presence in the councils of the King, especially since neither of Atanatar's sons, Narmacil nor Calmacil, showed great interest in affairs of state. Just a few short years after Narmacil succeeded his father as King, he conferred the office of Karma-kundo on his nephew Minalcar, who from that point effectively ruled Gondor, though officially doing so in the King's name.
In Christopher Tolkien's discussion of his father's notes for the unfinished portions of Aldarion and Erendis, he states that the Council of the Sceptre sought a regency to rule in Tar-Aldarion's place when he resolved to travel to Middle-earth, as no Numenorean King had ever left the island during his reign. The king's father, Meneldur, refused the regency when offered, and Meneldur's cousin, Hallatan of Hyarastorni, became regent instead. Later, mention is also made of Ancalimë serving as regent on more than one occasion while her father voyaged.