Theosophy, systematized chiefly by Helena Blavatsky in the late nineteenth century and elaborated in exhaustive clairvoyant detail by Charles Webster Leadbeater and Annie Besant in the early twentieth, describes the human being as composed of several interpenetrating bodies of increasingly subtle matter, of which the etheric double, or etheric body, is the one closest to and most tightly bound to the ordinary physical body. Leadbeater and Besant took over an existing Sanskrit term from Hindu Samkhya philosophy, linga sharira, and applied it specifically to this etheric layer, distinguishing it firmly from the term astral body, which they reserved for a separate, more emotionally charged layer of the human constitution corresponding to kama, desire.
Leadbeater's 1902 book _Man Visible and Invisible_ gave the etheric double its most detailed early published description in English, presenting it clairvoyantly as a faint, bluish-white aura closely tracing the contours of the physical form, composed of matter still physical in nature but finer than that perceived by the ordinary five senses. Its principal function, in Theosophical doctrine, is to absorb and distribute prana, the vital energy Theosophy adopted from Hindu sources, throughout the physical body, and to serve as the connecting medium between the physical body and the more emotionally and mentally active bodies described above it.