Kaoru Nagisa explains it in
Evangelion Episode 24.
"It is the holy region that must not be invaded by anyone. It is the light of the soul! You are aware of that, aware that the AT Field is the wall of the mind that everyone has."
We all (in Neon Genesis Evangelion) possess an A. T. Field, it is what keeps us separate from others and allows us to exist in a physical form.
A. T. Field
Abbreviation of "
Absolute Terror Field". An absolute domain deployed by the
Angels and
Eva that forms a
physical barrier. This field boasts enormous defensive
power, and is able to greatly diminish the power of all
weapons. The
Evas' ability to neutralize A.T.Fields is a major reason why they were viewed as the only weapon capable of countering the
Angels. However, not only Angels and Eva, but also
humans possess A.T.Fields, and it is these A.T.Fields that separate
humans from each other and give them their physical forms.
Kaoru Nagisa described A.T.Fields as "
the barrier of the heart that everybody has."
Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.