Mentioned by Kouzou Fuyutsuki in episode 26' as that possessed by Angels. It is written in the book of Genesis in the "Old Testament" that at the center of the Garden of Eden were the "Tree of Wisdom" and the "Tree of Life". Humans disobeyed God's command and ate of the "Fruit of Wisdom", but what if they had eaten of the "Fruit of Life" instead? Might this answer be the Angels? Further, it is also told in Genesis that they who eat of the fruit from the Tree of Life shall have immortal life.

Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.