Caesar originally meant "hairy", which suggests that the Iulii Caesares, a specific branch of the gens Iulia bearing this name, were conspicuous for having fine heads of hair (alternatively, given the Roman sense of humour, it could be that the Iulii Caesares were conspicuous for going bald). The first Emperor, Caesar Augustus, bore the name as a matter of course; born Gaius Octavius, he was posthumously adopted by Caesar in his will, and per Roman naming convention was renamed "Gaius Iulius Caesar Octavianus" (usually called "Octavian" during this stage of his life).
For political and personal reasons, Octavian chose to emphasise his relationship with Caesar by styling himself simply "Imperator Caesar" (whereto the Roman Senate added the honorific Augustus, "Majestic" or "Venerable", in 27 BC), without any of the other elements of his full name. His successor as Emperor, his nephew by blood Tiberius, also bore the name as a matter of course; born Tiberius Claudius Nero, he was adopted by Caesar Augustus on June 26, 4, as "Tiberius Iulius Caesar". The precedent was set: the Emperor designated his successor by adopting him and giving him the name "Caesar".
The fourth Emperor, Claudius I, was the first to don the purple and assume the name "Caesar" without actually being a Caesar at the time (he was, however, a member by blood of the Iulio-Claudian dynasty). The first to assume the purple and the name simultaneously without any real claim to either was the usurper Servius Sulpicius Galba, who donned the purple under the name "Servius Galba Imperator Caesar" in 68; he also helped solidify "Caesar" as the title of the designated heir by giving it to his own adopted heir, Lucius Calpurnius Piso Frugi Licinianus.
The next significant development in the title came one year later in 69, when the usurper Aulus Vitellius deposed the usurper Marcus Otho and donned the purple with the name "Aulus Vitellius Germanicus Imperator Augustus"; significantly, Vitellius did not at first adopt "Caesar" as part of his name, and may have intended to replace it with "Germanicus" (he bestowed the name "Germanicus" upon his own son that year). Nevertheless, Caesar had become such an integral part of the imperial dignity that its place was immediately restored by the fourth Emperor in 69, Titus Flavius Vespasianus, whose natural son, Titus Flavius Vespasianus became "Titus Caesar Vespasianus".
By this point, "Caesar"'s status had been regularised into that of a title given to the Emperor-designate (occasionally also with the honorific title Princeps Iuventutis, "Prince of Youth") and retained by him upon accession to the purple (e.g., Marcus Ulpius Traianus became Marcus Cocceius Nerva's designated heir as Caesar Nerva Traianus in October 97 and acceded on January 28, 98 as "Imperator Caesar Nerva Traianus Augustus"). After some variation among the earliest Emperors, the style of the Emperor-designate was NN. Caesar before accession and Imperator Caesar NN. Augustus after accession; starting with Marcus Aurelius Severus Alexander, it became popular to style the Emperor-designate as NN. Nobilissimus Caesar ("NN. Most Noble Caesar") rather than simply NN. Caesar.
On March 1, 293, Gaius Aurelius Valerius Diocletianus established the Tetrarchy, a system of rule by two senior Emperors and two junior sub-Emperors. The two coequal senior emperors were styled identically to previous Emperors, as Imperator Caesar NN. Pius Felix Invictus Augustus ("Elagabalus" had introduced the use of Pius Felix, "the Pious and Blessed", while Gaius Iulius Verus Maximinus "Thrax introduced the use of Invictus, "the Unconquered"), and were called the Augusti, while the two junior sub-Emperors were styled identically to previous Emperors-designate, as NN. Nobilissimus Caesar. Likewise, the junior sub-Emperors retained the title "Caesar" upon accession to the senior purple.
The Tetrarchy was quickly abandoned as a system, and the previous system of Emperors and Emperors-designate was restored, both in the Latin-speaking West (caesar) and the Greek-speaking East (kaisar); the collapse of the Roman Empire in the West led to "Caesar" falling into disuse there (although the so-called "Holy Roman Emperors" were called Kaiser in German, their correct title were in Latin usually as imperator augustus, without caesar), and most Western European languages use derivatives from imperator to refer to emperors (e.g., the English word "emperor"). In fact, in more recent history the word imperator came to replace the original meaning of imperator in Latin. In the East (in the so-called "Byzantine Empire") it suffered from gradual debasement.
In the East, the kaisar acquired a crown (without a cross) and was junior in rank to the Patriarch of Constantinople; as a result, this title was seen as a suitable one for a high prince of the blood, a regent, or an Emperor-designate (Emperors-designate were usually crowned as co-Emperors during their predecessors' reigns). The proliferation of individuals so titled prompted Aleksios I Komnenos to create the superior title sebastokratôr (a portmanteau word meaning "majestic ruler" derived from sebastos and autokratôr, the Greek equivalents of augustus and imperator) for his brother Isaakios. Both "Kaisar" and "Sebastokratôr" were reduced in degree when Manyhl I Komnenos introduced despotes as a suprior title; unlike the caesar and the sebastocrat, the despot had a territorial significance in addition to his degree of precedence.
The legacy of "Caesar" as an imperial title is reflected by the words for "Emperor" and "Empress" in many languages: