A.T. Field

An A.T. Field (short for "Absolute Terror Field") is an invincible barrier which Angels can generate. Conventional weapons such as those used by the combined military forces of the United Nations are essentially useless in penetrating an A.T. Field. Essentially, the only thing that can penetrate an A.T. Field is another A.T. Field, such as those created by the Evangelions. The Evangelions can create AT Fields equal to those of the Angels because the Evas are created from the cloned genetic material of Angels.

Besides the Evas, the only "conventional" weapon which has even marginal effect on the Angels is the N² Mine. The most powerful bombs ever created, N² mines are the pinnacle of the United Nation's arsenal. Still, they have only been moderately effective at stunning several of the weaker Angels temporarily, but against some Angels with strong A.T. Fields, even multiple N² mines have failed to do any damage whatsoever.

Later in the series, the Angel Kaworu Nagisa reveals that every living thing possesses an A.T. Field: Angels and Evangelions just have A.T. Fields so strong that they are easily detectable and can stop even physical attacks. "A.T. Fields" which regular humans generate are what literally holds individual beings' egos together, separating individuals. Ultimately this is the cause of loneliness and pain in the world, keeping individual people apart.

In the climax of The End of Evangelion, the Rei/Adam/Lilith hyper-being generates, along with the Mass Production Evangelions and Evangelion Unit-01, a massive "Anti A.T. Field" which neutralizes all A.T. Fields across the world, causing all of humanity to revert into the primordial soup of LCL, merging into a single super-organism in the climax of the Human Instrumentality Project.

Notes

• The phrase "Absolute Terror Field" is never actually mentioned within the series itself. It only appears in the OP.