Also called Criterion T.
A device due to Polish logician Alfred Tarski (1901-1983) and originally used in defining truth for a formal language, but later used (by American philosopher Donald Davidson (1930-2003)) to give an account of meaning in terms of truth.
The details are complex, but roughly: consider the sentence 'La neige est blanche' is true (in French) and if and only if snow is white. This is called a T-sentence for French and is in fact true.
A definition of truth for French will satisfy convention T if it entails all true T-sentences for French and no false ones. Truth here is taken to be relative to a language, as with Tarski's semantic theory of truth, and the 'definition' will be simply a conjunction of all the true T-sentences.
Tarski simply assumed that 'snow is white' is a translation of the French sentence, but Davidson tried to dispense with this assumption about meanings and provide a theory of meaning (for French in this case).
Davidson's initial proposal (modified later) was that a set of axioms might be constructed which would logically entail these T-sentences, and that such a set, with the rules of logic, would constitute a theory of meaning from French.
G Evans and J McDowell, eds, Truth and Meaning (1976)