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 Lilith. Lilith is maimed from the waist down, and through much of the series, she is impaled by the Lance of Longinus. This is believed to have been done to prevent her from regenerating. Lilith's flowing blood that is the source of L.C.L., which is collected and later used inside Entry Plugs.
This area of NERV is highly restricted, and only a handful of people know of its existence. Fewer still may enter it. Among those people are Gendo Ikari, Kouzou Fuyutsuki, and Ritsuko Akagi. Terminal Dogma is considered the innermost sanctum of NERV, and is the place where Gendo's version of the Human Instrumentality Project and the Third Impact would be triggered.
Although Terminal Dogma seems to have always been the true target of the Angels, only one; Tabris, manages to enter it.
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 much more detail, showing Misato and Shinji traveling though an enormous subterranean cavity with many organic elements. This is the inside of the Black Moon (an organic egg). The door to Terminal Dogma (aka Heaven's Door) has also been re-animated and is now much more complex and secure.