An Evangelion (エヴァンゲリオン) is a giant human being of near-godlike power, created by NERV from samples of the First Angel, Adam (although Unit-01 might be an exception). Evangelions were supposedly created to combat the Angel threat, although their most important function (and one of their greatest secrets) is to initiate Third Impact to evolve humanity further.
Despite being giant humans (in the series’ broader definition of the term “human”), Evangelions are often mistaken for giant robots, but as early as Episode 02 Evangelion Unit-01 is seen without its helmet on, revealing that Evangelions are clearly not robots, but cyborgs, living organic creatures with cybernetic machine and computer components grafted onto them. They are also covered from head to toe in restraining metal plates that hide their true organic form and serve to keep them under NERV’s control, and in part because they have pilots who sit in the cockpit-like Entry Plug.
Evangelions have been imbued with human souls. With the exception of Unit-00 and possibly the Mass Production Evas, this is always the soul of the pilot’s mother.