\begin{definition}[Definition:Axiom/Formal Systems/Axiom Schema]
Let $\LL$ be a formal language.
Part of defining a proof system $\mathscr P$ for $\LL$ is to specify its '''axiom schemata'''.
An '''axiom schema''' is a well-formed formula $\phi$ of $\LL$, except for it containing one or more variables which are ''outside'' $\LL$ itself.
This formula can then be used to represent an infinite number of individual axioms in one statement.
Namely, each of these variables is allowed to take a specified range of values, most commonly WFFs.
Each WFF $\psi$ that results from $\phi$ by a valid choice of values for all the variables is then an axiom of $\mathscr P$.
\end{definition}