Akâsa (Sk.). The subtle, supersensuous spiritual essence which pervades all space; the primordial substance erroneously identified with Ether. But it is to Ether what Spirit is to Matter, or Âtmâ to Kâma-rûpa. It is, in fact, the Universal Space in which lies inherent the eternal Ideation of the Universe in its ever-changing aspects on the planes of matter and objectivity, and from which radiates the First Logos, or expressed thought. This is why it is stated in the Purânas that Âkâsa has but one attribute, namely sound, for sound is but the translated symbol of Logos—“Speech” in its mystic sense. In the same sacrifice (the Jyotishtoma Agnishtoma) it is called the “God Âkâsa”. In these sacrificial mysteries Âkâsa is the all-directing ‘and omnipotent Deva who plays the part of Sadasya, the superintendent over the magical effects of the religious performance, and it had its own appointed Hotri (priest) in days of old, who took its name. The Âkâsa is the indispensable agent of every Krityâ (magical performance) religious or profane. The expression “to stir up the Brahmâ”, means to stir up the power which lies latent at the bottom of every magical operation, Vedic sacrifices being in fact nothing if not ceremonial magic. This power is the Âkâsa—in another aspect, Kundalini—occult electricity, the alkahest of the alchemists in one sense, or the universal solvent, the same anima mundi on the higher plane as the astral light is on the lower. “At the moment of the sacrifice the priest becomes imbued with the spirit of Brahmâ, is, for the time being, Brahmâ himself”. (Isis Unveiled).
Source: H.P.Blavatsky - The Theosophical Glossary