What is Ein Sof?
Ein Sof (Hebrew for "without end") is the Kabbalistic name for the infinite divine reality as it is in itself. It is the hidden God before any self-revelation, beyond all names and attributes. Because nothing can be said of it directly, the tradition speaks of it only by what it is not. From Ein Sof the ten sefirot, the divine attributes, are said to emanate.
Ein Sof vs the sefirot, Brahman, and the Godhead
Ein Sof is easy to confuse with three nearby ideas. First, it is not the sefirot. The sefirot are the ten powers through which the infinite becomes knowable and acts in the world, while Ein Sof is the concealed source they flow from. Second, it resembles the nirguna, or attributeless, Brahman of Advaita and the "One" of Neoplatonism, yet Kabbalah keeps it inside a Jewish frame of creation and Torah. Third, it is close to the apophatic "Godhead" of some Christian mystics, the God beyond God, which is why it sits near apophatic theology. The comparisons are real, but each tradition draws the lines its own way.
The Kabbalistic account
Within Kabbalah, the term took on its technical sense among 13th-century Kabbalists in Provence and Catalonia. Isaac the Blind and the circle around Azriel of Gerona used Ein Sof to name the limitless divine that lies beyond the sefirot. The Zohar, the central text of the tradition, appeared in late 13th-century Castile and deepened the picture. Its authorship is debated: tradition ascribes it to the 2nd-century sage Shimon bar Yochai, while most modern scholars point to Moses de León. In 16th-century Safed, Isaac Luria reshaped the whole system. He taught that Ein Sof first performed tzimtzum, a self-contraction or withdrawal, to clear a space in which a finite world could exist. The infinite is also called Or Ein Sof, the Limitless Light.
Where to encounter Ein Sof in the index
No single item in the index treats Ein Sof as its subject, but the idea stands behind several entries. It is the infinite that tzimtzum withdraws from, and the hidden source that the rest of Kabbalah unfolds. Readers drawn to the negative way will find the same instinct in apophatic theology and in the wider literature of mysticism. Comparisons with non-duality, emptiness, and Brahman show how different traditions name a reality that resists naming.
What it isn't
Ein Sof is not a being among beings, and not simply "God" in the everyday devotional sense. It is the divine considered as infinite and unconditioned, rather than the personal figure addressed in prayer, who in Kabbalah is met through the sefirot. It is also not a thing the mind can grasp. The point of the negative language is to stop the seeker from turning the infinite into one more object. Whether the comparisons with Vedanta, Buddhism, and Christian mysticism point to one reality or only to similar habits of thought is a question the traditions answer in their own ways, and not one this entry settles.