An abbreviation of Absolute Terror Field. The AT Field is initially portrayed as a barrier the Angels generate to create absolute territory. It is often deployed as a powerful defensive shield capable of repelling any physical attack.
The reason the Evangelion Units are the only weapons capable of opposing the Angels is largely due to their ability to generate and neutralize AT Fields. Neither the Strategic Self Defense Force nor the U.N. Forces have the ability to penetrate such fields, and despite their impressive arsenal of Bio-Chemical and N² weapons, they have proven incapable of combating the Angels.
All Angels and Evangelion Units posses AT Fields, although the strength of these fields vary. Later in the series an AT Field is also shown to be used offensively by Sahaqiel, the Tenth Angel.
At the end of the series, it is revealed that all humans generate AT Fields (which are referred to as the 'light of the Soul'). This in turn is what allows humanity to retain their separate forms and remain fractured among individuals. It is essentially the wall within one's mind that fosters a sense of incompleteness, and is therefore the cause of loneliness and pain (and thus is the root cause of mankind's suffering). To break down these AT Fields is the goal of the Human Instrumentality Project.
During the climax of The End of Evangelion, Lilith generates a global Anti-AT Field, thereby neutralizing the AT Fields of all humans on the planet. This has the effect of breaking down the walls that separate humanity into individuals, returning them to a unified statea sea of L.C.L.