(covers information from several alternate timelines)
Regent was a title given to one who ruled on behalf of another, typically because the actual ruler had not reached maturity, or 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")