Yechidah, the unique one, is the highest and innermost of the five levels of the soul, from the Hebrew for unity or the only one. It is the divine spark and the very point of union between the soul and God, the aspect in which the soul is utterly one with its Source and where it is held never to have descended at all. Like chayah it belongs to the expanded scheme associated with Lurianic Kabbalah and its transmission, and it is associated with Keter, the crown, and with the highest reach of Atziluth. It plays a substantial role in later Hasidic thought, particularly in Chabad, where the yechidah is the ground of the capacity for self-sacrifice. It should be said that these are terms within a religious and initiatory tradition with its own conditions of study; the atlas describes the scheme, which is public, and not the practice, which is not.
Hayyim Vital's Sha'ar HaGilgulim, the Gate of Reincarnations, ties yechidah to a person's individual soul-root and life-task rather than to any general human faculty, and the tradition is generally careful to say that even for its own most accomplished figures the level is rarely if ever fully actualized in life; it is present in potential rather than routinely exercised, unlike nefesh, which every living person is said to possess in full from birth. The Tanya's second chapter reads the yechidah as the basis for its claim that every Jew carries "an actual part of God above," language its author, Shneur Zalman of Liadi, draws directly from the Lurianic and Zoharic material on this highest level rather than coining independently.
Sources Physician of the Soul, Healer of the Cosmos: Isaac Luria and His Kabbalistic FellowshipLawrence Fine with Tanya (Likkutei Amarim)Shneur Zalman of Liady