LCL

An acronym for Link Connect Liquid, the amber fluid which fills the cylindrical "entry plugs" in which the Eva pilots sit while piloting. As the name implies, LCL is a transmission medium designed to convey the thought impressions of the pilot to the Eva Unit, allowing pilot and Unit to "synch" and thus operate in concert. LCL also provides oxygen to the lungs of the pilot; it is a liquid that can be "breathed," suggesting it may be desended from 20th-century U.S. Navy research which sought to create breathable liquids to replace gaseous oxygen for deep sea divers. Since, unlike gases, liquids cannot be compressed, such a medium provides protection from high-pressure environments. This implies that LCL may also serve to protect Eva pilots from physical shock in combat. LCL may have additional properties, which remain classified.

From Neon Genesis Evangelion #3 manga published by Viz Comics.