The word "emperor" in latin is imperator, which was a title given to any successful general in the roman empire.
Julius Caesar (100-44 B.C.) later adopted it as a permanent title, as did the subsequent Caesars.
Although it is not used in the New Testament, the new revised standard version (NRSV) uses the word to translate two different greek words, neither entirely accurately.
First, the greek kaisar, which came from the latin caesar.
Also the word basileus, which means King (e.g. 1 Peter 2:13,17).