The order issued by the government of
Japan in
New Tokyo-2 in episode 25' "
Air". This order means the
"abolishment of special legal protection for NERV and transfer of all command to the government of Japan". Order "
A-17" was issued when attempting to capture the Eighth
Angel, so all "A-" orders are thought to be
emergency actions related to
NERV.
Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.