In mathematical logic, Beth definability is a result that connects implicit definability of a property to its explicit definability. Specifically Beth definability states that the two senses of definability are equivalent.
First-order logic has the Beth definability property.
For first-order logic, the theorem states that, given a theory T in the language L' ⊇ L and a formula φ in L', then the following are equivalent:
Less formally: a property is implicitly definable in a theory in language L (via a formula φ of an extended language L') only if that property is explicitly definable in that theory (by formula ψ in the original language L).
Clearly the converse holds as well, so that we have an equivalence between implicit and explicit definability. That is, a "property" is explicitly definable with respect to a theory if and only if it is implicitly definable.
The theorem does not hold if the condition is restricted to finite models. We may have A ⊨ φ[''a''] if and only if B ⊨ φ[''a''] for all pairs A,B of finite models without there being any L-formula ψ equivalent to φ modulo T.
The result was first proven by Evert Willem Beth.