Definition:Axiom/Formal Systems/Axiom Schema

Definition
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 $\mathscr P$.

Examples
It was proved by in 1957 that ZFC and Peano arithmetic require an axiom schema.