Full Idea
A contextual definition shows how to analyse an expression in situ, by replacing a complete sentence (of a particular form) in which the expression occurs by another in which it does not.
Gist of Idea
Contextual definitions replace a complete sentence containing the expression
Source
A.George / D.J.Velleman (Philosophies of Mathematics [2002], Ch.2)
Book Reference
George,A/Velleman D.J.: 'Philosophies of Mathematics' [Blackwell 2002], p.29
A Reaction
This is a controversial procedure, which (according to Dummett) Frege originally accepted, and later rejected. It might not be the perfect definition that replacing just the expression would give you, but it is a promising step.