Full Idea
If φ contains no bound second-order variables, the corresponding comprehension axiom is said to be 'predicative'; otherwise it is 'impredicative'.
Gist of Idea
A comprehension axiom is 'predicative' if the formula has no bound second-order variables
Source
Øystein Linnebo (Plural Quantification Exposed [2003], §1)
Book Reference
-: 'Nous' [-], p.73
A Reaction
['Predicative' roughly means that a new predicate is created, and 'impredicative' means that it just uses existing predicates]