What does "regent" mean?

A regent is someone who rules for somebody else. Usually it is an uncle or mother who rules for a child, because the child is too young to be king, or for a mentally disabled king. The regent rules only until the child is old enough to take over. (Though sometimes the regent doesn't act this way, and instead kills the child or tries to keep ruling even after the child grows up).

Some famous examples of regents are Agrippina, ruling for her son Nero, Theoderic the Ostrogoth for his grandson Amalaric, Wu Chao, ruling for her son, Theophano ruling for her son Otto, Blanche of Castile ruling for her son Louis, and John, ruling for his brother Richard.