A few people have asked where the term Res Publica came from, here is some information from wikipedia:
Res publica in Ancient Rome
Dictionary lists the following meanings: "the common wealth, a commonwealth, state, republic (cf. civitas); also, civil affairs, administration, or power, etc.", which are elucidated below:
Basic meaning(s)
"Public property"
Res publica usually refers to a thing that is not considered to be private property (or, in Latin: res privata, the private matters of the society), but which is rather held in common by many people. For instance a park or garden in the city of Rome could either be "private property", or managed by the state, in which case it would be (part of the) res publica.
"The state" - "The 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 Imperium Romanum, 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.
"The (Roman) Republic"
Roman authors would also use the word res publica in the sense of the epoch when Rome was governed as a republic, that is the epoch 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" - "state organisation system" - "politics"
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.
