A rectangular
boat. According to the book of
Exodus in the
Old Testament,
Noah received a revelation from
God to load all
animals pair by pair into a huge Ark, thus saving all species from a world-wide
flood. The line spoken by
Kouzou Fuyutsuki in episode 26',
"Will it become the Ark to save humankind from the nothingness of Third Impact?" touches upon this legend.
Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.