Ain (Hebrew AIN, "not, nothing") is the third and the highest of the Three Veils of the Unmanifest, which are located above Kether on the Tree of Life and represent the inability of created beings to experience the divine in actuality. The other two Veils are Ain Soph and Ain Soph Aur.
Greer, John Michael. The New Encyclopedia of the Occult. St. Paul. MN. Llewellyn Worldwide. 2005. p. 9