Definition
A class K of structures of a signature σ is called an elementary class if there is a first-order theory T of signature σ, such that K consists of all models of T, i.e., of all σ-structures that satisfy T. If T can be chosen as a theory consisting of a single first-order sentence, then K is called a basic elementary class.
More generally, K is a pseudo-elementary class if there is a first-order theory T of a signature that extends σ, such that K consists of all σ-structures that are reducts to σ of models of T. In other words, a class K of σ-structures is pseudo-elementary iff there is an elementary class K' such that K consists of precisely the reducts to σ of the structures in K'.
For obvious reasons, elementary classes are also called axiomatizable in first-order logic, and basic elementary classes are called finitely axiomatizable in first-order logic. These definitions extend to other logics in the obvious way, but since the first-order case is by far the most important, axiomatizable implicitly refers to this case when no other logic is specified.
Read more about this topic: Elementary Class
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