Full Idea
An example of a recursive definition is 'y is an ancestor of x' is defined as 'y is a parent of x, or y is a parent of an ancestor of x'.
Gist of Idea
Recursive definition defines each instance from a previous instance
Source
Thomas Mautner (Penguin Dictionary of Philosophy [1996], 'definition')
Book Reference
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.126
A Reaction
From this example I guess that 'ancestor' means 'friend'. Or have I misunderstood? I think we need to define 'grand-parent' as well, and then offer the definition of 'ancestor' with the words 'and so on...'. Essentially, it is mathematical induction.