compactness theorem a theorem for first-order logic: if every finite subset of a given infinite theory T is consistent, then the whole theory is consistent. The result is an immediate consequence of the completeness theorem, for if the theory were not consistent, a contradiction, say ‘P and not-P’, would be provable from it. But the proof, being a finitary object, would use only finitely many axioms from T, so this finite subset of T would be inconsistent.
This proof of the compactness theorem is very general, showing that any language that has a sound and complete system of inference, where each rule allows only finitely many premises, satisfies the theorem. This is important because the theorem immediately implies that many familiar mathematical notions are not expressible in the language in question, notions like those of a finite set or a well-ordering relation.
The compactness theorem is important for other reasons as well. It is the most frequently applied result in the study of first-order model theory and has inspired interesting developments within set theory and its foundations by generating a search for infinitary languages that obey some analog of the theorem.
See also INFINITARY LOGIC. J.Ba.