Chayah, the living one, is one of the two highest and most transcendent levels of the soul, rarely fully manifest in ordinary waking life, and it belongs to a later stratum of the tradition than the three principal levels: the fivefold scheme is developed chiefly in the sixteenth century Kabbalah of Safed and above all in the system of Isaac Luria as transmitted by Hayyim Vital, though roots of it are found in earlier material. Chayah is associated with Atziluth, the world of emanation, and with Chokmah, and it is described as a level of soul that is not fully incarnate, touching the individual without being contained by them. The distinction between the three-level and five-level schemes is a good example of how Kabbalah develops: later systems do not usually replace earlier ones but extend them upward, and both remain in use.
The specific earlier material is the Tikkunei Zohar, a later layer of Zoharic literature composed separately from the Zohar's main body and generally dated somewhat later, into the fourteenth century, which already names all five levels, nefesh, ruach, neshamah, chayah and yechidah, though without the fuller systematic treatment Luria's school later gave them. The name itself is built on the Hebrew root for life and living, chai and chayim, the same root that names the toast lechayim, "to life"; later Hasidic writers, particularly in Chabad's Tanya and Likkutei Torah, describe chayah as the seat of a devotion so total it exceeds reasoned understanding, the point at which the soul's attachment to its source outruns what the intellect, seated lower in neshamah, can account for.
Sources Physician of the Soul, Healer of the Cosmos: Isaac Luria and His Kabbalistic FellowshipLawrence Fine with Tanya (Likkutei Amarim)Shneur Zalman of Liady