Briah is the second of the Four Worlds, the World of Creation and the first realm of separation, where the archetypes of Atziluth take their earliest form. It is the world of the Throne and of the great Archangels, the realm of pure creative mind, which receives the first, still-formless impress of what Atziluth emanates. The four worlds are used across Kabbalistic practice as a framework: the sefirot are held to exist in each world in a different mode, the four letters of the Tetragrammaton are distributed one to each, Briah taking the first Heh, and the four are correlated with the levels of the soul, so that neshamah belongs to Briah. This layering of one scheme onto another is characteristic of Kabbalah, which builds by correlating structures rather than by argument, and it is what gives the system its density.
The four-world scheme received its fullest early systematic treatment in Safed, first in Moses Cordovero's Pardes Rimonim of 1548, and then, following him, in Isaac Luria's system as transmitted by Hayyim Vital; the two Safed Kabbalists differ on details of how the worlds interpenetrate but share the basic four-fold structure inherited from the thirteenth and fourteenth century sources. The Golden Dawn's later London attributions gave Briah the element Water and its own colour scale, extending the medieval and Safed material into a ceremonial-magic correspondence table rather than replacing it.
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