
Regent was a title given to one who ruled on behalf of another (usually a monarch), typically because the actual ruler had not reached maturity, was absent, or infirm.
In 2267, a power struggle fomented by a Klingon agent left Leonard James Akaar as hereditary High Teer of the Ten Tribes from the time he was just hours old, following his father's death. His mother Eleen ruled as his regent, and signed a mining treaty with the Federation. (TOS: "Friday's Child")
Palamar was led by a regent during the 2370s. (DS9: "Business as Usual") The Evora were led by Regent Cuzar in 2375. (Star Trek: Insurrection)
In the mirror universe, the regent was the ruler of the Klingon-Cardassian Alliance. Worf was the Klingon regent in the 2370s. (DS9: "Shattered Mirror", "The Emperor's New Cloak")
Oxilon was the council regent of the Talaxians encountered by the USS Voyager in 2378. (VOY: "Homestead")