Full Idea
We have to include the term 'true', or some other semantic term, in the list of undefined terms of the meta-language, and to express fundamental properties of the notion of truth in a series of axioms.
Gist of Idea
We need an undefined term 'true' in the meta-language, specified by axioms
Source
Alfred Tarski (The Semantic Conception of Truth [1944], 10)
Book Reference
'Semantics and the Philosophy of Language', ed/tr. Linsky,Leonard [University of Illinois 1972], p.24
A Reaction
It sounds as if Tarski semantic theory gives truth for the object language, but then an axiomatic theory of truth is also needed for the metalanguage. Halbch and Horsten seem to want an axiomatic theory in the object language.