What do you mean? Republic comes from the Latin Res Publica ('the public thing'). Plato's Republic was called
Πολιτεία/"Politeía", meaning "city-state governance")
De re publica (Latin: On the commonwealth, see below) is a dialogue on Roman politics by Cicero, written in six books between 54 and 51 BC. It is written in the format of a Socratic dialogue in which Scipio Africanus Minor (who had died a few decades before Cicero was born, several centuries after Socrates' death) takes the role of a wise old man -- an obligatory part for the genre. Cicero's treatise was politically controversial: by choosing the format of a philosophical dialogue he avoided naming his political adversaries directly. Cicero employed various speakers to raise differing opinions in an attempt to make it more difficult for these adversaries to take him to task on what he had written.
What do you mean?
Regardless,
Roman authors would also use the word 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.