Full Idea
Roughly speaking, 'reflection principles' assert that anything true in V [the set hierarchy] falls short of characterising V in that it is true within some earlier level.
Gist of Idea
'Reflection principles' say the whole truth about sets can't be captured
Source
Peter Koellner (On the Question of Absolute Undecidability [2006], 2.1)
Book Reference
-: 'Philosophia Mathematica' [-], p.13