Terminal Dogma (also referred to as the L.C.L. Plant) is located deep beneath NERV Headquarters, and constitutes the lowest level of Central Dogma. It is a large, singular chamber flooded with L.C.L., with pillars of salt protruding its surface. It is accessible only through a highly secured gate aptly named Heaven's Door.
At the center of Terminal Dogma, stands a red cross with a crucified white giant called Lilith. Lilith is maimed from the waist down, and through much of the series is impaled by the Lance of Longinus. The lance is believed to prevent her from regenerating. Lilith's flowing blood is the source of L.C.L., which is collected and used inside Entry Plugs.
Terminal Dogma is the innermost sanctum of NERV Headquarters, and access to it is highly restricted. In fact, only a handful of people know of its existance, including Gendo Ikari, Kouzou Fuyutsuki, and Ritsuko Akagi. It is the place where Gendo's version of the Human Instrumentality Project and the Third Impact would be triggered as seen in The End of Evangelion.
Although Terminal Dogma has always been the true target of the Angels in their attempts to recover Adam, only Tabris manages to enter it. Upon realizing he was used by SEELE, he allows Shinji Ikari and Evangelion Unit-01 to kill him.
Based on Neon Genesis Evangelion Platinum: 06 DVD Booklet
Terminal Dogma is revealed much earlier in the plot of the 2008 feature film Evangelion: 1.0 You Are (Not) Alone than in the original series. It is now Misato Katsuragi, who leads Shinji Ikari into Terminal Dogma in order to convince him to pilot Evangelion Unit-01 against Ramiel.
Their descent into Terminal Dogma is also depicted in more detail, showing Misato and Shinji traveling though an organic subterranean cavity (the insides of the Black Moon). Misato also mentions that should an Angel ever penetrate this level of NERV Hearquarters, the facility will auto-destruct. And finally, the door to Terminal Dogma (aka Heaven's Door) has been re-animated and appears more secure.