Etheric double is, in Theosophy, the invisible of the ordinary, ordinary, physical body, which interpenetrates and beyond which it extends for a little was, forming the other finer bodies the aura. The term etheric is used because it is composed of that tenuous matter by the vibrations of which the sensation of light is conveyed to the eye. This matter, it must however be noted is not the omnipresent ether of space, but is composed of physical matter known as etheric, super-etheric, sub-atomic, and atomic. The term double is used to signify that it is an exact replica of the denser physical body.
The sense organs of the etheric double are the chaksams (see Chakras) and it is through these chaksams that the physical body is supplied with vitality necessary for its existence and well being during life. The etheric double thus plays the part of a conductor, and it also plays the part of a bridge between the physical and astral bodies, for with it man would have no communication with the astral world and hence neither thoughts nor feelings. Anaesthetics for instance, drive out the greater part of the double, and the subject is then impervious to pain. During sleep the double does not leave the physical body, and, indeed, in dreams the etheric part of the brain is extremely active, especially when, as is often the case, the dreams are caused by attendant characteristics such as noise.
Shortly after death the etheric double finally leaves the physical body though it does not move far away from that body, but is composed of the four subdivisions of physical matter as previously alluded to. With the decay of the latter, the double also decays, and thus to a clairvoyant a burying ground presents an unpleasant sight. A.G.H.
Source: 81, 150-151.