Full Idea
Given the reference (bedeutung) of an expression and a part of it, obviously the reference of the remaining part is not always determined. So we may not define a symbol or word by defining an expression in which it occurs, whose remaining parts are known
Gist of Idea
We can't define a word by defining an expression containing it, as the remaining parts are a problem
Source
Gottlob Frege (Grundgesetze der Arithmetik 2 (Basic Laws) [1903], §66)
Book Reference
Frege,Gottlob: 'The Frege Reader', ed/tr. Beaney,Michael [Blackwell 1997], p.268
A Reaction
Dummett cites this as Frege's rejection of contextual definitions, which he had employed in the Grundlagen. I take it not so much that they are wrong, as that Frege decided to set the bar a bit higher.