Regent
This depends on the context, but it normally means a person who rules or reigns. More specifically, it can mean:
Someone who rules in the absence of the actual ruler. If a king dies and the royal line leads to a child, for example, then a “regent” was often appointed to govern until the child comes of age.
Members of a board or governing body, especially in higher education. It’s very common for the boards of universities to be called the “Board of Regents” (in South Dakota, this refers to the state-wide group in charge of all state universities).
Why I Looked It Up
It came up in The Medici a few times. There were several regents at various times for various kingdoms in Renaissance Italy.
Also, in the science fiction TV show Babylon 5, a considerable plot point involved Milo Virinim, who was appointed as Regent after the Emperor died.
From the Babylon 5 wiki:
Regent is an old Centauri title of authority used in place of an Emperor when there is no clear line of succession or heir apparent.
(I’m not entirely clear why there was no new Emperor appointed in that case.)