Semantic concept formalized by the Polish-American mathematician and logician Alfred Tarski (1902-1983), although other thinkers had previously discussed the idea.
Truth theory concerns the truth-values of sentence structures in various formal logical languages. Tarski suggested a table by which these values could be determined (although he was less sure about whether the same rules could be applied to natural languages).
Also see: correspondence theory of truth
A Tarski, 'The Concept of Truth in Formalized Languages', Logic, Semantics and Metamathematics (1956)