Lilith is named for the first woman God created in Jewish mythology, who left the Garden of Eden after she refused to be subservient to Adam.
Lilith usurped Adam as the progenitor of life on Earth. All life on Earth is supposedly descended from this Angel, although it is never specified if this is in a biological sense or a metaphysical sense. Lilith's descendants possess the "Fruit of Wisdom," the source of human science and technology. The "Deleted Scenes" from episode 23 also reveal that Unit 01 is artificially derived from Lilith.
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http://en.wikipedia.org/wiki/Angel_%..._Evangelion%29