Full Idea
In semantic theories (e.g.Tarski's or Kripke's), a definition evades Tarski's Theorem by restricting the possible instances in the schema T[φ]↔φ to sentences of a proper sublanguage of the language formulating the equivalences.
Gist of Idea
Semantic theories avoid Tarski's Theorem by sticking to a sublanguage
Source
Volker Halbach (Axiomatic Theories of Truth [2011], 1)
Book Reference
Halbach,Volker: 'Axiomatic Theories of Truth' [CUP 2011], p.6
A Reaction
The schema says if it's true it's affirmable, and if it's affirmable it's true. The Liar Paradox is a key reason for imposing this restriction.
Related Idea
Idea 16295 Tarski proved that truth cannot be defined from within a given theory [Tarski, by Halbach]