According to Wikipedia
In mathematical logic, a theory (also called a formal theory) is a set of sentences in a formal language. Usually a deductive system is understood from context. An element $\phi\in T$ of a theory $T$ is then called an axiom of the theory, and any sentence that follows from the axioms ($T\vdash\phi$) is called a theorem of the theory.
a theory in a formal system is the set of axioms. But I don't know why I wrote it as the set of theorems in my old note. I am now sorting things out, so is the theory the set of axioms or the set of theorems?
Also Is the set of axioms required to be not deducible from each other under the set of inference rules? (i.e. to be minimal under the inference rules?)
I wonder if the set of theorems can be taken as a new set of axioms, which is equivalent to the original set of axioms?