AT Field is the abbreviation of Absolute Terror Field".
A physical barrier of absolute field put out by the Angels and EVA's. It boasts enormous defensive power, and it is able to greatly reduce the power of all attacking weapons.

The ability of EVA to neutralize the A.T.Field of the Angels is one major reason why EVA was considered as the only weapon against the Angels. However, the same A.T.Field does not exist only in the Angels or EVA's.

Human also possesses this field. With this A.T.Field, human is able to keep seperate from each other, and to stay in seperate physical forms.
Nagisa Kaworu explained A.T.Field as "a barrier of heart that everybody has".