wiki里面的解释：

Both emperors and kings are indicated as monarchs or sovereigns, but both emperor and empress are considered the higher monarchical titles. In as much as there is a strict definition of emperor, it is that an emperor has no relations implying the superiority of any other ruler and typically rules over more than one nation. Therefore, a king might be obliged to pay tribute to another ruler,[3] or be restrained in his actions in some unequal fashion, but an emperor should in theory be completely free of such restraints. However, monarchs heading empires have not always used the title in all contexts—the British sovereign did not assume the title Empress of the British Empire even during the incorporation of India, though she was declared Empress of India.


【 在 lxqrc 的大作中提到: 】
: 一直没找到满意的答案，其中一篇是四十年前发表在  历史教学  上的，大意是中国学者认为叫王国合适就是王国，习惯称皇帝就是皇帝
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FROM 114.253.33.*