Full Idea
Attempts to define a term frequently elicit necessary but not sufficient conditions for membership of its extension. This is called the 'X problem', as in 'kill' means 'cause to die' plus X.
Clarification
The 'extension' is the objects picked out by the concept
Gist of Idea
Definitions often give necessary but not sufficient conditions for an extension
Source
Jerry A. Fodor (LOT 2 [2008], Ch.2.1 n3)
Book Reference
Fodor,Jerry A.: 'LOT 2: the Language of Thought Revisited' [OUP 2008], p.27
A Reaction
Fodor is one of the great sceptics about definition. I just don't see why we have to have totally successful definitions before we can accept the process as a worthwhile endeavour.