Expression (mathematics): Well formed formula, Equation, Syntax (logic), Formal semantics, language, Combinatory logic, Defined and undefined, (programming), Formula

Gödel Numbering: Mathematical logic, Function (mathematics), Well formed formula, Formal language, Natural...

Realizability: Proof Theory, Formula, Proof, Well Formed Interpretation (Logic)

Decidability (logic): Logical system, Propositional logic, validity, consequence, Syntax (logic), Formal semantics