What's the difference between a king and an emperor?

Balaji Viswanathan answers:

An Emperor is the highest power in the region he occupies. For most practical purposes they were in a world of their own (Emperors of China, India, Persia, and others have often seen themselves as the rulers of the world). If there is a bigger power in the vicinity (whether political or religious), it would be silly to call him an emperor. Kings are not necessarily the biggest powers and they often pay tributes to other powers (emperors, pope, or other kings). While the title of king is primarily political, the title of emperor often makes one the head of the religion too.

While a King rules one fairly homogenous territory (called a nation or kingdom), emperors often wield power over a fairly heterogeneous territory (ruler of many nations). Persians are usually assumed to have originated the term "King of Kings" (Shahanshah).

In the ancient/medieval world, emperors had special significance. Europe, for instance, had only one Emperor at any point, and the control was said to transfer in a quasi linear way (Translatio imperii). In India, only the Samrāṭ/Chakravartin can perform the Rajasuya sacrifice.

In the past, there would also be specific priority as a part of protocol that would treat an emperor different from the kings. In the modern day the terms have been abused enough to render the difference meaningless.

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