A logical formula is called *universally valid* if it is true in every possible domain and with every possible interpretation, inside that domain, of non-constant symbols used in the formula. To say that it can be proved means that there exists a formal proof of that formula which uses only the logical axioms and rules of inference adopted in some particular formalisation of first-order predicate calculus.

The branch of mathematical logic that deals with what is true in different domains and under different interpretations is Model theory; the branch that deals with what can be formally proved is [Proof theory]?. **The completeness theorem, therefore, establishes a fundamental connection between what can be proved and what is (universally) true; between model theory and proof theory; between semantics and syntax in mathematical logic.** It should not, however, be misinterpreted as obliterating the difference between these two concepts; in fact, another celebrated result by the same author, Goedels Incompleteness Theorem, shows that there are inherent limitations in what can be achieved with formal proofs in mathematics.

*Some of the older text that is to be integrated when more is added to the above:*

A statement is called *universally valid* if it is true in every domain in which the axioms hold. To cleanly state Gödel's completeness theorem, one therefore has to refer to an underlying set theory in order to clarify what the word "domain" in the previous sentence means.

The theorem can be seen as a justification of the logical axioms and inference rules of first-order logic. The rules are "complete" in the sense that they are strong enough to prove every universally valid statement. It was already known earlier that only universally valid statements can be proven in first-order logic.

- Kurt Gödel, "Über die Vollständigkeit des Logikkalküls", doctoral dissertation, University Of Vienna, 1929. This dissertation is the original source of the proof of the completeness theorem.
- Kurt Gödel, "Die Vollständigkeit der Axiome des logischen Funktionen-kalküls", Monatshefte für Mathematik und Physik 37 (1930), 349-360. This article contains the same material as the doctoral dissertation, in a rewritten and shortened form. The proofs are more brief, the explanations more succinct, and the lengthy introduction has been omitted.
- Vilnis Detlovs and Karlis Podnieks, "Introduction to mathematical logic", http://www.ltn.lv/~podnieks/