Full Idea
An 'impredicative' definition is one that uses the terms being defined in order to give the definition; in some way the definition is then circular.
Gist of Idea
An 'impredicative' definition seems circular, because it uses the term being defined
Source
Michèle Friend (Introducing the Philosophy of Mathematics [2007], Glossary)
Book Reference
Friend,Michèle: 'Introducing the Philosophy of Mathematics' [Acumen 2007], p.172
A Reaction
There has been a big controversy in the philosophy of mathematics over these. Shapiro gives the definition of 'village idiot' (which probably mentions 'village') as an example.