Constantine I was using "caesar" as late as 313 AD on his bronze coinage. Here's my RIC VII Arles 9. IMP(ERATOR) C(AESAR) CONSTANTINVS P(IVS) F(ELIX) AVG(VSTVS) Already during Constantine's time "caesar" was falling out of use. It wasn't a normal legend for him; it had been used occasionally earlier, then briefly at Rome/Ostia and Arles after his victory over Maxentius (who had also been using it, presumably as an appeal to tradition). Magnentius would then also revive this archaic legend at Rome in 350 AD. Here he is on RIC VIII Rome 179, with a slightly fuller "CAE" abbrevation. IMP(ERATOR) CAE(SAR) MAGNENTIVS AVG(VSTVS) The AVG(VSTVS) title of course continued quite a bit later. I'm not sure if Magnentius was the last to use CAESAR. The later emperors tended to use "DN ..." rather than "IMP C ...".
The title "augustus" was used on coins well into the early modern era and beyond. The (mostly German) Holy Roman Empire saw itself as the direct successor of the Roman emperors, and its rulers used the respective titles. Here are some examples: The reverse legend of this coin abbreviates the phrease "Rudolphi II Romanorum imperator augustus permissa fieri decreto," which translates as 'made with permission of Rudolph II, Roman emperor and augustus:' County of Waldeck, under Franz III, Wilhelm Ernst, Christian, and Volrad, ½ Batzen, 1595 AD, Niederwildung mint. Obv: FRA. WE. C W. [C. I. WAL], helmeted shield with eight-rayed star; below, moneyer’s mark of Kaspar Huxer: crossed hooks and arrow. Rev: RVDOL. II. IM. AVG. P [.F. DEC]; 9-5; imperial orb with value mark Z. 19mm, 0.98g. Ref: Slg. Hennig 62; Grönegreß 1986, 292. Ex Ernst Otto Horn (1880–1945) collection; ex Staatliche Kunstsammlung Dresden (1945–2010). This one has "Ferdinandus III, deo gratia Romanorum imperator, semper augustus" ('Ferdinand III, by God's grace Roman emperor and forever augustus'): Augsburg, Imperial City, AR ½ Batzen, 1637 AD. Obv: AVGVSTA • VINDELICORVM; arms of Augsburg: “Pyr” on column; in fields, 16-37; horseshoe mark of the moneyer family Holeisen. Rev: FERDINAND • III • D • G• ROM • IMP • S • AVG; crowned imperial double eagle with central imperial orb inscribed 2. 17.5mm, 1.14g. Ref: Forster: Erzeugnisse der Stempelschneiderkunst (1910), no. 268. Same title for emperor Maximilian II: Brunswick, City, AR groschen (1/24 taler), 1573 AD. Obv: MON. NO. ARG. REIP. BRVNS; shield with arms of Brunswick (lion l.); above, moneyer’s mark (wheel). Rev: MAXI: D: G: ROM: IM: SEM: AV*; imperial orb with 24; 7-3; 22 mm, 2.04g. Ref: Jesse 76.
I changed my mind. I believe that Julian II was the last figure who had NOB C inscribed on his coins during the latter reign of Constantius II, which would place it a few years after Magnentius was killed.
Julian II was Caesar under Constantius II until his promotion in 361 AD. When Valentinian took over, his underlings were given the title Augustus even when it was obvious that they were not as 'Augustus' as he was. The distinction between the two levels had been causing troubles since Caesar was first given to an adult not related to the Augustus (even by adoption). Eliminating it made sense IMO. While Caesar as a title lived on into the 20th century (Kaisar Wilhelm II died in 1941) and I have known half a dozen people named Caesar, I believe the answer that best works here is Julian II. DN IVLIANVS NOB CAES Were there any later Augusti who also used Caesar as part of their string of titles?
The coronation of Charlemagne in 800 was at first not accepted by the byzantine (Eastern Roman) Emperor, who still claimed the title and the right to appoint a western roman Caesar for himself. However, after some battles, they reached an agreement. Charlemagne got the title "Imperator ruling Rome" for himself and his sucessors (The holy Roman emperors). In return the byzantine emperor Michael I. got Venice, Dalmatia and Istria. From this point on the Holy Roman Emperors used the titel Imperator and Caesar (Kaiser) until Francis II, put down the title in 1806. Beside of the Holy Roman Emperors, there is this type for Zeno and Leo Caesar https://www.acsearch.info/search.html?id=117760
Leo Gabalas used the term Caesar on his Rhodian coinage up to the 1250s. Beyond that, I don't believe it was used again on Roman/Byzantine coins As for the longest continually used title from Ancient Rome: I own a coin from 1328 with the title Imperator. The anonymous Basilikon of Andronikos II and Michael IX featured the legend AVTOKPATO PЄC IOMЄON Left coin: Anonymous Basilikon Right coin: Named Basilikon Autokrator is Greek for Imperator. The title was used up to at least 1395 on the Stavraton of Manuel II. Beyond that, I don't believe it appeared on Roman/Byzantine coinage. After the 1395 date, emperors referred to themselves with only non-traditional roman titles such as Basileus or Despot. I personally would not include the HRE in any sort of discussion about continuation from the Roman Empire.
Julian II is the last to see any sort of large coinage output while he was still Caesar under Constantius II, although I believe others held the title, albeit not necessarily on coinage. Patricius (son of Aspar) was briefly Caesar under Leo I, but no coinage exists for him as such. Leo II I think was elevated straight to co-Augustus with his grandfather, although I could have sworn I saw one solidus of Leo I with Leo II as Caesar. Can't find it now, maybe I'm just imagining things. At any rate, there are joint reign "ZENO ET LEO NOV CAES" - although on paper Leo II was the appointed heir and thus "senior" emperor, I'm sure that it would not have been hard to talk him into taking the title of Caesar under his father, given that he was about six years old. https://www.acsearch.info/search.ht...de=1&fr=1&it=1&es=1&ot=1¤cy=usd&order=0 Of course there were claimants to the title of Augustus until the fall of the Holy Roman Empire (and Kaiser/Czar were extensions of that claim), although I believe it was somewhere around around 8th/9th century that the Byzantines dropped AVG on coinage, of course exacerbated by the fact that the average mint worker was likely illiterate and the rigid abbreviations conventions of the late Roman Empire had gone out the window. This is around the same time that Charlemagne and his immediate successors issued some rare coins with the traditional IMP AVG titulary. https://www.acsearch.info/search.html?id=1216390
There was a Western Turk king of either Kabul or Ghazni in the 8th century who styled himself Fromo Kesaro, phonetically "Roman Caesar", on his coins. "Fromo Kesaro" is inscribed on the coin below in Bactrian, using cursive Greek letters (obverse margin, 1h to 6h). It is speculated that he was granted the honorific title of Caesar by the Byzantine emperor for his assistance in battle against the Arabs (image courtesy CNG). As already noted, the Russian title Czar/ Tsar is actually "Caesar", originally granted by Justinian II to the Bulgarian emperor in 705. Fromo Kesaro was probably a contemporary of Leo III or Constantine V.
It's interesting, and confusing, how the term "caesar" was used in the later empire... More title than name, but certainly not a rank since it was retained by the heir once he became emperor, but only then(?) would he be *called* caesar by the masses. I'm not sure there's any modern equivalent. I'm always tempted to think of the emperor as "augustus", but I suppose that's really just an honorfic and "caesar" (cf tsar) seems closer to a title of "emperor", despite also being the title of the heir(s) in waiting.
The title Caesar entered the Germanic languages already in the first century. Hence, the German title Kaiser preserved much of the original Classical Latin pronunciation of the word.
Off topic, but how Latin sounded is also an interesting question. Today's Italian and the other Romance languages have changed a lot since then. One of the most similar languages to the original Latin is probably Rheto-Romance. The language comes from a small region in Switzerland where the inhabitants are particularly stubborn. If you want to hear it, here is Franz Hohler, a swiss writer from this region, speaking Rheto-Romance: The Sardinian language is also very close to Latin, but usually they speak faster. It would be interesting to know which of the two would have been understood in ancient Rome.