L.C.L. is the liquid that fills an Entry Plug once a pilot is on board. It is viscous and has an unpleasant odor similar to that of blood. Once inside an Entry Plug, the L.C.L. is electrically charged, altering its molecular configuration and turning its color from a murky orange to a clear translucent.
Through this process L.C.L. manifests the ability to mentally link a human pilot with the Evangelion Unit. This neural-link connection is made via the A10 Nerve, which travels from the brain stem and is distributed in the frontal association cortex of the cerebral cortex. L.C.L. also directly oxygenates the pilot's lungs, as well as serves to shelter the pilot from psychological and physical trauma. Because of this, it is often said to resemble the amniotic fluid of a mother's womb (although that fluid does not oxygenate the lungs).
L.C.L. turns out to be the harvested blood of Lilith, the Second Angel, and is collected and processed at a highly secured facility called the L.C.L. Plant (also known as Terminal Dogma), located at the lowest level of NERV Headquarters.
Based on Neon Genesis Evangelion Platinum: 05 DVD Booklet