Contact Experiment

"Contact experiment" (接触実験, sesshoku jikken) is an ambiguous term used in the show to describe certain experimental procedures conducted between a human and a Seed or Seed-derived entity with the intention of separating, fusing, and/or cojoining the body and soul of it's participants. Three are known to have been explicitly performed: The first, about which little is explicitly known yet is understood to have birthed Kaworu Nagisa, was conducted between a human and Adam, playing a role in the initiation of Second Impact. This was eventually followed by two other contact experiments, of a different nature, involved in the early development of the Evangelions and with separate outcomes.

Adam & Second Impact
The Katsuragi Expedition Team undertook a mysterious procedure on September 13, 2000, which one scientist referred to as "the contact experiment with the donor". This "contact" was apparently physical, and resulted in DNA "diving" into Adam and "physically fusing" resulting in Adam's soul being transferred into the body of Kaworu and Adam's body being reduced to an Embryonic state.

Therefore, Tabris' birth via this joining of Lilin genes and Adam's soul, has been forwarded as the direct impetus for Second Impact. The original identity of the donor is unknown. In terms of the effects this is plainly seen: Adam's Soul was separated from its Body and the body was lost for a time having been reduced to an embryonic state.

Yui Ikari & Unit-01
In 2004, Yui Ikari participated in a contact experiment with Evangelion Unit-01 at her own request. In what appeared to be an accident to onlookers, but was in fact planned by her, Yui's body dissolved into LCL and her soul was absorbed into the Eva's core. The subsequent attempt to salvage her soul back into a reconstructed body failed due to Yui's desire to remain in the Eva; with Lilith's soul instead being implanted into these reconstructions henceforth.

Kyoko Zeppelin Sohryu and Unit-02
At an unspecified date, Kyoko Zeppelin Sohryu undertook a contact experiment with Evangelion Unit-02. Unlike Yui's experiment, Kyoko's does not cause her soul to be completely absorbed into Unit-02. Rather, only an aspect of her soul resides in Unit-02 - the part that loves her daughter above all else. However, due to her soul being split, Kyoko goes insane and later commits suicide, leaving Asuka without a mother. During Episode 25', both the insane and maternal aspects of Kyoko are heard talking to Asuka at the same time, suggesting that the remainder of Kyoko's soul was salvaged into Unit-02 at some point after her suicide.