Adam
What can be considered as "The First Angel". Adam is the source of everything since the second impact and the reason that NERV, SEELE and the Evangelions exist. However, Adam could become the cause of the Third Impact.
Whenever an Angel comes into contact with Adam to unite, the whole human race will be destroyed. The only force capable of stopping this from happening are the Evangelions.
NERVE
An organization directly attached to the United Nations, the existence of which is not known to the general public. NERV is also the executive branch of SEELE.
Possessing super-advanced technology, it was the one responsible for the development and possession of the Evangelion units. Their base is located underground and is called the Geo-Front.
Their supreme commander is Gendo Ikari.
NERV was originally formed as the Geihrin Project in 2003 AD, but was later dissolved by Chairman Keel Lorenz and the organization known as NERV was born in 2010 AD.
Evangelion
The Multi-Purpose Fighting Machine which NERV has built in order to combat the giant Angels. These giant robots function through a cerebral-nerve link with their operators. The pilots connect through the A10 neuron system of the brain. They can connect through their plug suits and the medium of liquid known as LCL.
Normally, they draw on external power provided through huge umbilical cables. However, when necessary, they can run on internal power for a limited number of time. Five minutes if they are in standby mode and one minute if they are full-active mode. This shortage in power has been solved when Eva 01 managed to engulf an Angel's S2 Engine, making Unit 01's power supply unlimited whether connect through an external cable or not.
Currently, the Evangelion robot is the only machine with enough ability to cancel out an enemy's AT field. They use a wide range of weapons from a Prog-Knife to a Pulsitron Rifle.
In the course of the series, Ryouji Kaji explains the importance of the Evangelions. Kaji said that if at any time an Angel comes in contact with Adam, who is beneath NERV headquarters, the disaster known as Third Impact woudl occur. The only ones with the power to stop the Angels are the Evangelions.
Dr. Ristuko Akagi also stressed that the "armor" of the Evangelion is actually Bindings which let them control the Evas. Without the bindings, the Evangelions are very difficult to control (if fact, beyond human control). She also stressed, that Evangelions are man made by other men and that they have no human souls within them.
Man did something and found God. When they picked-up God, God became angry and punished man (Second Impact). God disappeared. So man, tried to make God with his own hands: Adam. And man created himself in Eva.
“Adam’s alter ego and Lilith’s minion” - Kaoru
Angels
Possessing a variety of forms that go beyond the concepts of living and non-living entities, they are the enemy, threatening mankind's exsistence with superscientific capabilities (weapons). They cannot be harmed by conventional weaponry because of their AT field. Their objective is to be one with Adam. Once this is accomplished, humanity will perish in an event called The Third Impact.
Geo-Front
NERV's headquarters is in Geo-Front, the huge underground complex underneath Tokyo-3. The complex was designed as the last line of defense against the Angels. Built to last even if it is severed from contact from the outside world, the Geo-Front is a whole world within itself complete will all the things required to sustain life.
More than 89% of the facilities are hidden underground.
Tokyo-3
A planned city, under construction near Lake Ashino, in Hakone. In preparation for the coming of the Angels, Tokyo-3 was made into a fortress city. Under the camouflage of skyscrapers are facilities known as "Armament Buildings". These contain missles and other heavy fire power weaponry, as well as other weapons used by the Evangelions.
Entrada Plug
Used by Evangelion pilots to increase synchronization with their respective Evangelion units. These are uniquely designed suits that shrink to fit their wearer's size.
These plug suits also enable the pilot to retain body heat for a period of time (a sort of space suit). These plug suits can also be specially designed to protect the pilot from extreme heat as well.
LCL Fluid
Also known as the goo. This is the liquid that enables the pilots of synchronize with their respective Evangelion units.
This fluid is first ejected into the entry plug and then is energize, when this happens, this fluid becomes similar to a liquid type of breathable oxygen with the pilots can use. The LCL fluid can also be used as a barrier against a psychic attack.
However, Shinji always comments that the LCL fluid smells like blood...hmmm...