Akasha is the ether or space tattva, the subtlest and first of the five gross elements of the Indian scheme, the principle of space, vibration and sound, out of which the other four elements are said to unfold. In the classical analysis ether is associated with the sense of hearing and with the medium in which all things exist and move; in the subtle physiology it is linked to the region of the throat and to sound and the spoken word. As the most rarefied element it is regarded as the closest to the unmanifest, the ground from which air, fire, water and earth successively densify.
Akasha entered Western esoteric vocabulary through Theosophy and through Rama Prasad's Nature's Finer Forces of 1890 and the Golden Dawn, which represented it as a black or indigo egg in the tattva meditations from 1888; the Theosophical notion of the "akashic records" is a further modern extension. The ether tattva itself, as the element of space and sound, belongs to the ancient Indian doctrine.
Sources Classical Samkhya: An Interpretation of Its History and MeaningGerald James Larson with Nature's Finer Forces: The Science of Breath and the Philosophy of the TattvasRama Prasad