After Caesar (which simply happened to be his family name) Octavius took the title Augustus which means better, more, augmented. And most Roman emperors and strong men took the title Augustus thereafter. Their second in command was given the title Caesar/Kaiser. Please correct me if this description is flawed!
But then why didn't the Germans use the title Augustus rather than Caesar for their word for Emperor?
Caesar came first, and Caesar remained.
We observe that the first really big contact between Germanic tribes and Rome took place when Gaius Iulius Caesar was campaigning 'in Gaul'.
We observe also that under his adopted nephew Octavian the largest forays of Rome into Germanic lands was undertaken, bordering on colonising and provincialising Germany up to the Elbe river. The well known Battle of the Teutoburg Forest took place when Octavian was imperator, when we call him Augustus.
But what was his name then?
As a consequence of Roman customs, society, and personal preference, Augustus (/ɔːˈɡʌstəs, əˈɡʌs-/; Classical Latin: [awˈɡʊstʊs]) was known by many names throughout his life:
- Gaius Octavius Thurinus (/ɒkˈteɪviəs/): He received his birth name, after his biological father, in 63 BC. "Gaius" was his praenomen, "Octavius" was his nomen, and "Thurinus" was his cognomen. Later, his rival Mark Antony used the name "Thurinus" as an insult, to which Augustus replied, surprised that "using his old name was thought to be an insult".54
- Gaius Julius Caesar: After he was adopted by Julius Caesar, he adopted Caesar's name in accordance with Roman naming conventions. While he dropped all references to the gens Octavia, people colloquially added the epithet Octavianus to his legal name, either to differentiate him from his adoptive father or to highlight his more modest origins. Modern historians refer to him using the anglicized form "Octavian" (/ɒkˈteɪviən/) between 44 BC and 27 BC.
- Gaius Julius Caesar Divi Filius: Two years after his adoption, he founded the Temple of Caesar additionally adding the title Divi Filius ("Son of the Divine") to his name in attempt to strengthen his political ties to Caesar's former soldiers, following the deification of Caesar.
- Imperator Caesar Divi Filius: From 38 BC, Octavian opted to use Imperator, the title by which troops hailed their leader after military success. His name is roughly translated as "Commander Caesar, Son of the Divine".
- Imperator Caesar Divi Filius Augustus: Following his 31 BC defeat of Mark Antony and Cleopatra, partly on his own insistence, the Roman Senate granted him the additional name, "Augustus", which he added to his previous names thereafter. Historians use this name to refer to him from 27 BC until his death in AD 14.
Augustus successor then renamed himself Tiberius Julius Caesar.
That means that from an almost equivalent to 'first contact' for the next 3 generations, the Germanic tribes when asking legionariries "who's your boss?" they would have heard at least some kind of "Caesar!".
In both cases, we also take note that the persons bearing that name would react quite peculiar when asked by anyone: "Are you a king?" Namely with "ONonononono, I am not a king, I am something different, the restorer of the res publicas!"
That it was around that time the name was borrowed into Germanic languages is evidenced by linguistics and language development. Caesar/Kaiser is one of the oldest Latin loanwords into Germanic languages. Loaning must have happened around that time that the mono-archicical biggest honcho of the Romans was always called some form of Caesar as the history of the sound of the German word proves: Shortly after this time the Latin language itself changed and the German loanword preserved the older sound:
Probably the oldest German loan word from Latin is Kaiser. The word goes back to the family name Caesar or its most famous bearer Gaius Iulius Caesar (100-44 B.C.) and was used as title under the following rulers. The word first came into Germanic and then further into German. The ahd. keisur (also -or, -ar) is probably also still used as title, but at the same time already as deonomastikon, as an appellative derived from the name with the meaning *ruler. Similarly, under the impression of an overpowering ruler, the Slavs later gave the name of Charlemagne, i.e. ahd. Karal / Karl, borrowed and used as ruler designation (Russian. король, Czech. král, Polish. król etc. for *King). But also to the Russian. царь *Zar is based on Caesar, it is a loan from got. kaisar, which goes back to Caesar via greek καῖσαρ . The sound laws allow here an approximate dating of the loan of Caesar into Germanic: It could not take place later than in the 2nd century, because until then the Diphthong ae was still spoken in Caesar, which also appears in ahd. keisur or got. kaisar; also the k- pronunciation was still preserved in the sound of Caesar, only in the 6th/7th century was k- palatalized, thus spoken as [ts].
- Kurt Gärtner: "Lehnübersetzung und Lehnbedeutung vs. Lehnwort: Zu den Entlehnungen aus dem Lateinischen und Französischen in das mittelalterliche Deutsch", in Jens Braarvig & Markham J. Geller (Eds): "Studies in Multilingualism, Lingua Franca and Lingua Sacra", Edition Open Access: Berlin, 2018.
Thus the oldest German loanword was well established when the Romans started to devise ever more complicated stylings for their 'emperor'. This more legalistic terminology of the Romans in Latin was then indeed adopted as well by the Germnic tribes that tried their luck at renovatio imperii. Charlemange as emperor was titled:
Karolus serenissimus Augustus a Deo coronatus magnus pacificus imperator Romanum gubernans imperium
That illustrates that Germans have really borrowed all available titles: Caesar, Imperator, Augustus.
While Augustus as a title is only used in Latin forms of titles and as a word declined in meaning and understandability, Caesar/Kaiser remained a very scarce given name. But August became common from the 14th century onward and besides also signifying a month it also took on a row of unfortunate connotations. Imperator remained a decidedly foreign word. Kaiser was the first, the most enduring, the most popular of these forms. And when a German speaking peasant would ask what the various long Latin titles (containing some form of Augustus) of Germanic emperors would come to mean, the answer would again be "Kaiser!"
That means that in medieval times the full title was a legal thing, and as such Latin dominated. Only the colloquial translation into the tongue of the people was still the old germanised word. That this got somewhat out of hand in being complicated is evidenced by the long-form of the titles of the kaisers of Austria. 'Long' as in three pages long.
When the German Empire was founded in 1871 the nationalist mood and complicated power politics within the Reich and in regard to Austria called for a simpler solution. A title that would still allow kings within Germany but also elevate the Prussian Hohenzollern king above the others. Thus the function of Federal President of the Reich got named with the word for 'biggest honcho in the empire' from 2000 years ago: Kaiser.
Why do people assume that the title of Roman Emperors was a single word?
Is President Trump's title "President" or "President of the United States"? Since the title is abbreviated POTUs I would guess it is a phrase.
The president of the United States (POTUS)[B] is the head of state and head of government of the United States of America. The president directs the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces.
So Wikipedia seems to think the title is a phrase.
Is the title of Elizabeth II a single word or a phrase?
Since 1953 her title in the United Kingdom has had an English version and a Latin version, which are not, repeat not, exact translations:
In English: Elizabeth the Second, by the Grace of God of the United Kingdom of Great Britain and Northern Ireland and of Her other Realms and Territories Queen, Head of the Commonwealth, Defender of the Faith18
In Latin: Elizabeth II, Dei Gratia Britanniarum Regnorumque Suorum Ceterorum Regina, Consortionis Populorum Princeps, Fidei Defensor Oceania
So many present titles are phrases. Some may object that by "title" they specifically mean the single word that describes the rank or position or function of the leader, and not the complete phrase that describes both the type of leadership and also the group that is led.
Even accepting that argument, there are plenty of examples of ranks, positions, or functions that are described by phrases and not by single words.
For example, "Chairman of the Board of Directors of General Motors", "Chairman of the Presidium of the Supreme Soviet of the Union of Soviet Socialistic Republics", "Grand Duke of Lithuania", "Vice Admiral", "Field Marshal", "First Class Boy", "Grand Prince of Kiev", "Grand Master of the Teutonic Knights", "Cardinal Deacon", "Nagusa Nagast" meaning "King of Kings", etc., etc.
The early Roman emperors used a number of separate powers, titles, and honors. Their main titles included Imperator, Caesar, Augustus, and Princeps.
There are examples of an Imperator Caesar Augustus having authority over a lesser ruler or designated successor with the title of Imperator Caesar, so one could say that in the Roman Empire Imperator Caesar Augustus meant "emperor", or "senior emperor", and Imperator Caesar meant "emperor's heir" or "junior emperor".
Therefore, I am somewhat skeptical that a single Latin word meant "emperor", and tend to think that the three word phrase Imperator Caesar Augustus meant emperor.
The titles used by the Holy Roman Emperors varied t lot over time.
By about 1190 the title of a man elected emperor but not yet crowned in Rome by the pope was:
Romanorum Rex semper augustus
"King of the Romans always Emperor/Imperial".
And after being crowned in Rome by the Pope the title became:
Romanorum Imperator et semper augustus
"Emperor of the Romans and always Emperor/Imperial".
In the later middle ages semper augustus was mistranslated in the German versions of the title.
Romischer kayser zu allen zeyten merer des reichs
There Caesar instead of imperator or augustus, or all three together, is translated into German as kaiser, and semper augustus, meaning "always emperor" or "always imperial", is mistranslated into German as zu allen zeyten merer des reichs, meaning "in all times enlarger of the realm", or "perpetual enlarger of the empire".
The last major change in the imperial title came when emperors stopped going to Rome to be crowned and took the imperial title immediately after election and coronation in Germany.
Electus Romanorum Imperator semper Augustus, ac Germaniae… Rex,
"Elected Emperor of the Romans, Always Emperor (or always Imperial), and King in (or of) germany".
erwählter Römischer Kayser, zu allen Zeiten Mehrer des Reichs, in Germanien… König,
"Elected Emperor of the Romans, in all times Enlarger of the Realm, King in Germany".
In my opinion emperor, empereur, emperador, imperador, etc. based on imperator, and kaiser, tsar, etc. based on Caesar are equally valid or invalid words for emperor, since they are all single words based on single words that are part of the full phrase imperator caesar augustus which might be considered necessary to fully give the title of the Roman emperor. The fact that the Holy Roman Emperors believed themselves to be Roman emperors is why they often and usually used both imperator and augustus in their Latin titles.
Of course the single words like "emperor", kaiser, and tsar, etc., etc., are quite sufficient for the lesser 18th, 19th and 20th century monarchs of Russia, France, Austria, Haiti, Mexico, Brazil, Germany, India, Bulgaria, and Central Africa, showing that in rank they were some sort of "sub emperors", at least one rank lower than the Roman, "Byzantine" and Holy Roman Emperors.
There is a very detailed discussion of this whole complex here:
I will not repeat the whole argument. The gist of the discussion is that the use of derivatives of "Caesar" by German and Russian potentates in the middle ages has a lot to do with the "give unto Caesar" pericope in the New Testament. But read it for yourself.