If we want to answer this question, we need to understand two things first. A) where does the title of emperor derive from? And B) what significance does the title hold?

The first question can be answered rather easily, at least in European context, as the title of emperor is a more than less direct successor of the Roman Principate. When Julius Caesar died in 44 BC, the Principate (as the early phase of the Roman Empire) was developed by his adopted son, Octavian, who replaced the res publica with a de facto monarchy. After Octavian was given the honorary name "Augustus" by the Senate in 27 BC, he styled himself as Imperator Caesar Augustus. It is important to note here that the names "Caesar" and "Augustus" were not yet considered imperial titles in their own right; the only official titular denotation in his regal name at the time was Imperator (which originally referred to the holder of an imperium - supreme military or civic command).

His successor as emperor, his stepson Tiberius, also bore the name as a matter of course; born Tiberius Claudius Nero, he was adopted by Caesar Augustus on 26 June 4 AD, as "Tiberius Julius Caesar". The precedent was set: the Emperor designated his successor by adopting him and giving him the name "Caesar". The fourth Emperor, Claudius, was the first to assume the name "Caesar" upon accession, without having been adopted by the previous emperor; this marked the transformation of the name into an independent imperial title. The name "Augustus" went through a similar development over the decades.

This was concluded during and immediately after the "Year of the Four Emperors" with the accession of Emperor Vespasian in 69 AD. Vespasian's regal name Imperator Caesar Augustus can be seen as the precedent standard regal name for all following Roman emperors. The names "Caesar" and "Augustus", which until then had denoted a family affiliation of all preceding emperors to the Julio-Claudian dynasty of Julius Caesar, now became titular designations. This was in part certainly motivated by Vespasian to legitimise his own rule, as he wasn't affiliated with the Julio-Claudians, hoping to emulate some of his famous predecessors.

Etymologically, the English word 'emperor' and its equivalent in Romanic languages (e.g. empereur, emperador, imperador) are derived from the Roman word "Imperator", whereas its equivalent in Germanic and Slavic languages (e.g. Kaiser, Kaizer, Tsar, Cesarz) are derived from "Caesar". However, the connections between our modern understanding of the title of emperor and the Roman Principate go beyond mere linguistics, the Principate is the political and religious foundation of it as well - which conveniently brings us to my second point; the title's significance.