I've found in Wikipedia an interesting formal definition of what a propositional logic (propositional calculus) "exactly" is. Unfortunately it does not cite its source. I wonder which is the original source for this definition, does someone recall it?
I have checked some typical literature (Monk1976, Barwise1993, Matiyasevich1994, Ebbinghaus1994, Rautenberg2010, Manin2010, Girard2011, Pudlak2013), however I could not find a comparable definition. Ebbinghaus comes somewhat close, but it is still far from being this one.