Definition:Theorem/Formal System

Definition
Let $\mathcal F$ be a formal system consisting of a formal language with deductive apparatus $\mathcal D$.

A theorem is a well-formed formula in $\mathcal F$ which can be deduced from the axioms and/or axiom schemata of $\mathcal D$ by means of its rules of inference.

Also see
As can be seen, this is equivalent to the definition in the context of logic if one defines the apparatus of logic as a formal system.