LCL

LCL is a yellow-tinted, 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, LCL 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.

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 production 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, the AT Fields of all human beings collapse and all people around the world revert to puddles of LCL.

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.