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".
Read more about Res Publica: In Ancient Rome, Calques