Full Idea
We must specify the words or concepts which we wish to use in defining the notion of truth; and we must also give the formal rules to which the definition should conform. More generally, we must describe the formal structure of the language.
Gist of Idea
For a definition we need the words or concepts used, the rules, and the structure of the language
Source
Alfred Tarski (The Semantic Conception of Truth [1944], 01)
Book Reference
'Semantics and the Philosophy of Language', ed/tr. Linsky,Leonard [University of Illinois 1972], p.14
A Reaction
This, of course, is a highly formal view of how definition should be achieved, offered in anticipation of one of the most famous definitions in logic (of truth, by Tarski). Normally we assume English and classical logic.