Full Idea
Usually in a definition the definiens (definition) can replace the definiendum (expression defined), but in a 'contextual definition' only the whole statement containing the definiens can replace the whole statement containing the definiendum.
Gist of Idea
'Contextual definitions' replace whole statements, not just expressions
Source
Thomas Mautner (Penguin Dictionary of Philosophy [1996], 'definition')
Book Reference
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.126
A Reaction
These definitions are crucial to Frege's enterprise in the 'Grundlagen'. Logicians always want to achieve definition with a single neat operation, but in ordinary language we talk around a definition, giving a variety of possibilities (as in teaching).