(Sē KIKAN) A power generation system explained by the super-solenoid theory advocated by Dr. Katsuragi. When an Evangelion is equipped with this system, its operational time limit is extended to infinity. All the Angels have Sē engines, and Eva-01 acquired one when it devoured the 14th Angel. Moreover, all Evangelion Mass-Production Models from Eva-05 onward are also equipped with Sē engines and thus do not need umbilical cables.

Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.

Please note that the '2' in S2 is in superscript, as in 'Sē', in the original program, however to ease the discovery of this write-up the node title is listed as simply S2.