Full Idea
Frege appeals to a general principle that nothing should be defined in terms of that to which it is conceptually prior.
Gist of Idea
Nothing should be defined in terms of that to which it is conceptually prior
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §64) by Michael Dummett - Frege philosophy of mathematics Ch.3
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.33
A Reaction
The point is that the terms of the definition would depend on the thing being defined. But of all the elusive concepts, that of 'conceptual priority' is one of the slipperiest. An example is the question of precedence between 'parallel' and 'direction'.