Emperor

An Emperor is a monarch, and usually the sovereign ruler of an empire or another type of imperial realm. Empress, the female equivalent, may indicate an Emperor's wife, mother, or a woman who rules in her own right.
Both Emperors and Kings are monarchs, but 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, 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.