A large-scale
catastrophe predicted to follow (
Giant)
First Impact and
Second Impact. It was officially explained that
Third Impact would be caused by contact between an
Angel and
Adam kept underground below
Tokyo-3, and that the
Evangelions and
NERV existed to prevent this from happening. However, the objective of the
Instrumentality Project was actually to
initiate Third Impact.
Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.