Axiom schema

In mathematical logic, an axiom schema (plural: axiom schemata or axiom schemas) generalizes the notion of axiom.

Formal definition
An axiom schema is a formula in the metalanguage of an axiomatic system, in which one or more schematic variables appear. These variables, which are metalinguistic constructs, stand for any term or subformula of the system, which may or may not be required to satisfy certain conditions. Often, such conditions require that certain variables be free, or that certain variables not appear in the subformula or term.

Finite axiomatization
Given that the number of possible subformulas or terms that can be inserted in place of a schematic variable is infinite, an axiom schema stands for an infinite class or set of axioms. This set can often be defined recursively. A theory that can be axiomatized without schemata is said to be finitely axiomatizable.

Examples
Two well known instances of axiom schemata are the: Czesław Ryll-Nardzewski proved that Peano arithmetic cannot be finitely axiomatized, and Richard Montague proved that ZFC cannot be finitely axiomatized. Hence, the axiom schemata cannot be eliminated from these theories. This is also the case for quite a few other axiomatic theories in mathematics, philosophy, linguistics, etc.
 * induction schema that is part of Peano's axioms for the arithmetic of the natural numbers;
 * axiom schema of replacement that is part of the standard ZFC axiomatization of set theory.

Finitely axiomatized theories
All theorems of ZFC are also theorems of von Neumann–Bernays–Gödel set theory, but the latter can be finitely axiomatized. The set theory New Foundations can be finitely axiomatized through the notion of stratification.

In higher-order logic
Schematic variables in first-order logic are usually trivially eliminable in second-order logic, because a schematic variable is often a placeholder for any property or relation over the individuals of the theory. This is the case with the schemata of Induction and Replacement mentioned above. Higher-order logic allows quantified variables to range over all possible properties or relations.