A person who reigns or rules jointly with another person, sharing royal power.
From co- (together) + reigner (one who reigns), from Old French regner, Latin regnare meaning to rule. The prefix co- was added to indicate shared rulership.
Medieval and early modern kingdoms sometimes had coreigeners to ensure peaceful succession—a king would crown his son as coreigner years before his death, guaranteeing a smooth transition and preventing civil wars over the throne.
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