Full Idea
A truth theory is 'conservative' if the addition of the truth predicate does not add any new theorems to the base theory.
Gist of Idea
A theory is 'conservative' if it adds no new theorems to its base theory
Source
report of Volker Halbach (Axiomatic Theories of Truth [2011], 6 Df 6.6) by PG - Db (ideas)
Book Reference
Halbach,Volker: 'Axiomatic Theories of Truth' [CUP 2011], p.44
A Reaction
Halbach presents the definition more formally, and this is my attempt at getting it into plain English. Halbach uses Peano Arithmetic as his base theory, but set theory is also sometimes used.