Adam (also referred to as the Giant of Light) is the First Angel, and is discovered inside a subterranean cavity called the White Moon under Antarctica in 2000 A.D.
Following the discovery, SEELE scientists set out to reduce Adam to an embryonic state in their efforts to minimize the potential damage of the Angels as prophesized by the Dead Sea Scrolls. Although they succeed, the process triggers the planetary disaster known as the Second Impact. This too however was prophesized by the Dead Sea Scrolls and was expected by SEELE.
Although Adam is considered to be an Angel and is classified as such, he is actually one of two Progenitors of Life (the other is Lilith). While humanity (the Lilim) are believed to have been spawned of Lilith, the Angels were spawned of Adam. The Angels that attack New Tokyo-3 do so in their efforts to be reunited with Adam, which they believe is held inside Terminal Dogma.
After Adam's angelic form is destroyed during the Second Impact, he is preserved in a dormant embryo and stored at NERV-03 in Germany. During the transfer of Evangelion Unit-02 to Japan, the embryo is smuggled by Ryoji Kaji into the possession of NERV Commander Gendo Ikari.
Gendo then has the embryo grafted onto the palm of his hand. It is believed he does this in an attempt to keep the embryo secure, but could also be an effort to gain Adam's regenerative abilities. The embryo is invaluable to Gendo because it is a critical part of his version of the Human Instrumentality Project. He eventually merges the embryo with Rei Ayanami, who ultimately awakens Lilith as seen in The End of Evangelion.