Full Idea
When a definition contains a quantifier whose range includes the very entity being defined, the definition is said to be 'impredicative'.
Gist of Idea
Impredicative definitions quantify over the thing being defined
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.35
A Reaction
Presumably they are 'impredicative' because they do not predicate a new quality in the definiens, but make use of the qualities already known.