LCL
LCL is an amber-colored, translucent (and frankly, Tang™-like) liquid, which allows an Eva pilot to mentally link with their Evangelion Unit. The Entry Plug of an Eva Unit, containing its cockpit, is completely flooded with LCL, and, because it is oxygenated, upon being submerged Eva pilots can "breathe" the liquid (similar to real-life experiments involving liquid breathing).
Upon Evangelion activation, an electrical current is run through the LCL and it undergoes a phase shift, after which its density, opacity and viscosity appears to approach that of air. However, the LCL remains in a liquid phase and does not undergo a transition: the pilots appear to be surrounded by air, however occasionally air bubbles will float away from their mouths, revealing that they are still in a liquid medium.
It is stated several times in the series that LCL smells "like blood". LCL is, in fact, the blood of the Second Angel, Lilith, which is restrained in the deepest level of Nerv HQ, "Terminal Dogma". The "LCL Plant" in Terminal Dogma is actually an entire lake formed from the blood of Lilith.
LCL shares similar properties with the hypothetical "primordial ooze" from which life on Earth first evolved. This is because non-Angel life on Earth actually originally evolved from the LCL spread by Lilith when she landed on Earth. During Third Impact, Lilith's Anti A.T. Field causes the A.T. Fields of human beings and all other Lilith-based life to collapse, reverting their bodies to puddles of LCL.
LCL apparently has sources in addition to Lilith's body. In Episode 20, Shinji's body reforms from LCL within the core of EVA-01. LCL is also seen escaping EVA-00's core in e23' and EVA-01's (again) in e26'.
An amber-colored fluid, visually identical to LCL, escapes from Leliel's body (the ultra-flat "shadow") when the bullets of EVA-01's handgun strike [e16].
Notes
It is unknown what the initials "LCL" stand for, but we do know what they don't mean. According to the Death and Rebirth theatrical program (special edition):
Incidentally, the widely circulated idea that L.C.L is the abbreviation of "Link Connected Liquid" is incorrect.
Nowhere does it say what it does mean. Evangelion Chronicle suggests that one of the Ls stands for "Lilith".
