There have been many attempts to define truth in terms of  correspondence,  coherence or other notions.  However, it is far from clear that truth is a definable notion. In formal settings satisfying certain natural conditions, Tarski’s theorem on the undefinability of the truth predicate shows that a definition of a truth predicate requires resources that go beyond those of the formal language for which truth is going to be defined. In these cases definitional approaches to truth have to fail. By contrast, the axiomatic approach does not presuppose that truth can be defined. Instead, a formal language is expanded by a new primitive predicate for truth or satisfaction, and axioms for that predicate are then laid down. This approach by itself does not preclude the possibility that the truth predicate is definable, although in many cases it can be shown that the truth predicate is not definable.