Res publica is a Latin phrase, loosely meaning ‘public affair’. It is the root of the word ‘republic’, and the word ‘commonwealth’ has traditionally been used as a synonym for it; however translations vary widely according to the context. ‘Res’ is a nominative singular Latin noun for a substantive or concrete thing – as opposed to ‘spes’, which means something unreal or ethereal – and ‘publica’ is an attributive adjective meaning ‘of and/or pertaining to the state or the public’. Hence a literal translation is, ‘the public thing/affair’.
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Public property
Res publica usually is something held in common by many people. For instance a park or garden in the city of Rome could either be ‘private property’ (res privata), or managed by the state, in which case it would be part of the res publica.
The state or commonwealth
Taking everything together that is of public interest leads to the connotation that the ‘res publica’ in general equals ‘the state’. For Romans this equalled of course also the Roman Empire, and all its interests, so Res Publica could as well refer to the Roman Empire as a whole (regardless of whether it was governed as a republic or under imperial reign). In this context scholars suggest commonwealth as a more accurate and neutral translation of the term, while neither implying republican nor imperial connotations, just a reference to the state as a whole. But even translating res publica as ‘republic’ when it clearly refers to the Roman Empire under Imperial reign occurs (see quotes below).
The Roman Republic
Roman authors would also use the phrase res publica in the sense of the era when Rome was governed as a republic, that is the era between the Roman Kingdom and the Roman Empire. So in this case res publica does distinctly not refer to the Roman Empire, but to what is generally described as the Roman Republic.
Public affairs or institutions
Res publica could also be used in a generic meaning, referring to "public affairs" and/or the general system of government of a state. In this usage res publica translated the Greek concept politeia (which originally meant the state organisation of a city-state). Also, for a Roman politician engaging himself in the res publica, a translation can often be the even more generic "being occupied in politics".
Other uses
Even when limited to its "political" connotations, the meanings of the term res publica in ancient Rome are diverse and multi-layered, and differing from the Greek politeia in many ways (that is: from the several interwoven meanings the word politeia had). However, it is also the customary Latin translation of politeia; the modern name of Plato's The Republic comes from this usage.
In some contexts the "state organisation system" meaning of res publica derives into something like "constitution", although "constitution", properly speaking, is a much more modern concept. Ancient Romans would use the expression "Twelve Tables" instead of res publica, when referring to their constitution at the time of the "republic", and the "inalterable laws installed by the divine Augustus", for their equivalent of a constitution in the era of the early Empire.
After the Roman Empire collapsed in the West, the idea of res publica disappeared, as foreign to the barbarians of the Migrations Period: whenever Gregory of Tours refers to res publica, it is the Eastern Empire of which he is speaking.
Quotations
The translations of the quotations below are copied without alteration from existing non-copyrighted material. Other translations might differ, but they all serve to illustrate the many aspects of the res publica concept in Ancient Rome. The Latin original texts are given concurrently with the translations, in order to show that only the context of the text allows to interpret the res publica concept in each instance.
From these examples it also follows that probably there was also a gradual shift of meaning of the res publica concept throughout the Roman era: the "(Roman) Republic" connotation of res publica is something that rather occurs with retrospect to a closed period (so less appararent in Cicero's time, who never knew the era of the Emperors, and could only compare with the epoch of the Kings); on the other hand the translation of the Greek "politeia" concept appears to have nearly completely worn off in late antiquity.
Cicero
Cicero's De re publica (this translates as "about the res publica"), a treatise of the 1st century BC in Socratic dialogue format, takes the res publica as its subject. The differing interpretations and translations of the title of that work are discussed in the "De re publica" article. The expression res publica is of course used several times throughout the work too. The quotes below aim at demonstrating that within any translation of Cicero's work differing English translations of the term res publica need to be used, according to context, in order to make sense. The quotes are taken from the Latin text at "The Latin Library" (chapter numbering follows this text), from C. D. Yonge's translation at gutenberg.org (2nd column) and from Francis Barham's translation at "The Online Library of Liberty" (3rd column).
When Cicero refers to the Greek authors (pointing at the "politeia" concept):
When pointing at the Roman context:
The translation shows that the meaning of res publica can differ even within the same paragraph...
Pliny the Elder
When Pliny dedicates his Naturalis Historiae to his friend Emperor Vespasian in the first century, he uses the word res publica (Latin from LacusCurtius website / 1601 Philemon Holland translation from http://penelope.uchicago.edu/holland/index.html / 1855 John Bostock translation from the Perseus website):
When under an Emperor, that is Vespasian or his predecessors, Pliny was not talking about the Roman Republic, but used "commonwealth"/"republic" in the meaning of "the state". Of course the ambiguity of Rome still considering itself formally, or just "pro forma", a republic throughout the era of the principate, when a monarchic rule had already de facto been established, adds to the complexity of translating "res publica" in this context.
Tacitus
As another example of the complexities of the meaning of the word res publica one can cite Tacitus, who in the early 2nd century described in his Annals how the first Emperors, like Tiberius in the year Augustus had died (AD 14), sought to preserve all institutions of the Res publica completely intact (Latin and translation as available at the Perseus Project):
... while Tacitus complained in the same writing that at the same time the res publica went astray for good because not a single soul seemed to care any more:
The least that can be said is that the two quotes above (like so many passages in Tacitus' writings) are a translator's minefield:
Nonetheless it can only be admired in Tacitus how, with some judicially chosen words, he most poignantly and to the point describes the transition from "(overdue) remnants of the republic" to "actual Imperial reign, already established in the minds of people".
In his book Germania, Tacitus also uses res publica in the context of the Germanic "barbarian" society. Here the word is used to convey the generic meaning of "public affair" or "the commonwealth" (in contrast to the private or family life) without the Roman connotations of republicanism. This is illustrated in the following text (Latin text and English translation from the Perseus Project):
Augustine
Augustine of Hippo uses the word res publica several times throughout his work The City of God, in which he comments, in the early 5th century on several Greek and Roman authors. Again, the standard translations of the expression "res publica" are multiple throughout the work. Examples taken from the Latin text at "The Latin Library", English translation from the version available at "New Advent"
Meaning "the (Roman) state" in general:
Note that in this quote Augustine does not use the expression imperium Romanum ("the Roman empire") as a synonym to "the era when Rome was governed by emperors". Compare also to the 2nd quote from Tacitus above: there an expression different from res publica and imperium Romanum is used for referring to "the (Roman) State" in general.
Meaning "the Roman Republic" as era with a distinct form of state organisation, from the same book:
Calques
Later calques of Res publica: