Full Idea
The standard sense of a 'contextual definition' permits the eliminating of the defined expression, by transforming any sentence containing it into an equivalent one not containing it.
Gist of Idea
A contextual definition permits the elimination of the expression by a substitution
Source
Michael Dummett (Frege philosophy of mathematics [1991], Ch.11)
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.134
A Reaction
So the whole definition might be eliminated by a single word, which is not equivalent to the target word, which is embedded in the original expression. Clearly contextual definitions have some problems