Atziluth is the highest of the Four Worlds of the Kabbalistic cosmos, the World of Emanation or of the archetypes. It is pure, undifferentiated divine radiance, the realm of the Sefirot in their most exalted being and of the divine Names, where the Godhead and its outflowing are held to be still one. The four-world scheme is not biblical but is derived from a reading of Isaiah 43:7, in which three verbs, bara to create, yatzar to form and asah to make, are taken as naming three descending activities, with emanation added above them; the derivation is set out in the medieval sources and is one of the clearest cases of a Kabbalistic structure built out of close reading of a single verse.
Each world is also correlated with one letter of the Tetragrammaton, Atziluth with the initial Yod, and, from 1888, with a Golden Dawn elemental and colour-scale attribution, Fire and the order's so-called King scale, part of the fourfold set of correspondence tables it built onto the older medieval structure in London. In the Lurianic Kabbalah systematized at Safed in the sixteenth century, Atziluth is not quite the topmost reality either: Isaac Luria's system places a prior configuration, Adam Kadmon, the Primordial Man, above all four worlds, as the first patterning of divine light after the initial contraction (tzimtzum), so that even emanation's world is already a step down from the system's true beginning.
Sources The ZoharTraditionally attributed to Moses de Leon with The Golden Dawn: The Original Account of the Teachings, Rites, and Ceremonies of the Hermetic Order of the Golden Dawn, 6th editionIsrael Regardie, Physician of the Soul, Healer of the Cosmos: Isaac Luria and His Kabbalistic FellowshipLawrence Fine