The difference between emperor and king

I wonder why we say "TENNOH," who is the symbol in Japan, EMPEROR.
There are many address like king, but I think few people are called emperor.

I looked into it.

At first, I found the difference between emperor and other addresses.

That is

Emeperor or empress are more major than pope, and pope is more major than king or queen or sultan. So, in the difinition, emperor is very very high level person.It difined that emperor is the reader of country which govern not only his own country but also many other countries.

Second, I decided on inquiring why tennoh was settled to translate emperor in English. The reason is this.
In Meiji era, Japanese governers wanted to conquer a loads of Asian countries so that they might dicide their TENNOH was emperor in English.

So, in this difinition, absolutely TENNOH is a king but is called EMPEROR.
Hmm... history is interesting!

address　呼称
major 偉い
sultan　イスラム国王