Also known as the
AT Field, this is a
forcefield first seen to be generated by the
Angels and
Evas in the
anime series "
Neon Genesis Evangelion". According to
NERV's explanations to the Japanese government, it is because normal weapons cannot penetrate the AT Field that the Evangelions are necessary to repel the Angels.
Later in the series -- and particularly in End of Evangelion -- the AT Field is revealed to have more universal importance.