The final calamity following the First Impact and the Second Impact. Publically, it is believed to occur once an Angel comes into contact with Adam beneath New Tokyo-3, and foretold to be on a scale that will wipe out mankind.
While public perception is that NERV is utilizing the Evangelion Units to combat the Angels in an effort to prevent the Third Impact from occurring, in truth NERV is charged with eliminating the Angels only to execute their own version of the event in the form of the Human Instrumentality Project as dictated by SEELE.
The Third Impact is triggered by the union of Adam and Lilith, resulting in the creation of a supreme being that would succeed all existing forms of life on Earth. The goal of SEELE and NERV is to prevent the Angels from initiating the event so that humankind may do so instead, thus fulfilling the goal of the Human Instrumentality Project. For a human Third Impact to occur, the Angels must be defeated, as they apparently guard the Tree of Life. With the death of Tabris, all Angels are dead, and the Tree of Life becomes accessible.
The Third Impact is a global Anti-AT Field event, dissolving all AT Fields and causing all life to revert to L.C.L. All Souls are then drawn to the Seed of Life for Complementation into the progenitor entity, thus achieving a form of divinity. The Third Impact is considered mankind's Armageddon, a genocide that is depicted in its terrifying glory during the climax of The End of Evangelion.
As the human version of the Third Impact begins, Shinji Ikari is placed at the head of the Tree of Life, composed of himself in Evangelion Unit-01 and Evangelion Units
Based on Neon Genesis Evangelion Platinum: 06 DVD Booklet