A-801 is the order code issued by the government of Japan in The End of Evangelion, abolishing the special legal protections held by NERV and transferring it's holdings back to the Japanese government. Orders starting with 'A' seem to be emergency orders relating directly to NERV. Order A-17 was issued following the capture of Sandalphon, the Eighth Angel.
Following the issue of order A-801, the Japanese government (under the influence of SEELE), authorized the use of the Strategic Self Defense Force in an effort to secure NERV Headquarters.