A queen regnant (plural: queens regnant) is a female monarch, equivalent in rank and title to a king, who reigns suo jure (in her own right) over a realm known as a "kingdom"; as opposed to a queen consort, who is the wife of a reigning king; or a queen regent, who is the guardian of a child monarch and rules pro tempore in the child's stead, be it de jure in sharing power or de facto in ruling alone. Such sovereign would be styled as woman king.

Both Maria of Anjou and Maria Theresa were crowned ‘king’ (Rex Hungariae) and assumed masculine titles. Thus, in nomenclature, Maria Theresa was archduke and king; normally, however, she was styled as queen.