Lińga is the Sanskrit term in the Samkhya system for the characteristic "mark" of the individual reincarnating entity. The Lińga Śarīra is the subtle body or vehicle of consciousness in later Samkhya, Vedanta, and Yoga. In Theosophy the Lińga Śarīra or Linga Sharira became the name for the invisible double of the human body; also referred to as the etheric body, etheric double, astral body (which has two different meanings), doppelgänger, bioplasmic body, pranamayakosa, and so on in New Age writings.
In the Classical Samkhya system of Isvarakrsna (ca. fourth century c.e.), the Lińga is the characteristic mark or feature of the transmigrating entity. Karana or "instrument" is a synonymous term. It consists of the tattvas from buddhi (also called mahat) down to the five organs of sense and five of activity (buddindriya or jñānendriya, and karmendriya respectively) and the five subtle elements (tanmatras), and is propelled by past-life tendencies, or bhavas. 
The Samkhyakarika says —
- "The subtle body (linga), previously arisen, unconfined, constant, inclusive of the great one (mahat) etc , through the subtle elements, not having enjoyment, transmigrates, (because of) being endowed with bhavas ("conditions" or "dispositions")
- As a picture (does) not (exist) without a support or as a shadow (does) not (exist) without a post etc; so to the instrument (linga or karana does not exist without that which is specific (i.e. a subtle body)"
- vv 60-81, transl. Gerald Larson, Classical Samkhya p.268
The idea of a Linga Sarira or Sukshma Sarira (subtle body) was also adoped by Vedanta and Yoga philosophy, and from there, in the 19th century, by the Theosophy of Madame Blavatsky, where it was given a somewhat different meaning.
In Theosophy, the Linga Sarira corresponds not to the entire reincranating entity, or the subtle body as opposed to the physical body, but to only one of the seven principles, the Astral body, which is part of the lower quartenary . According to Blavatsky, it is the eidolon of the Greeks, and is above the physical plane, from which it is separated by a Laya center.
According to theosophical philosophy, the Linga Sarira serves as a model or matrix of the human body form, thus it is often called the "double" since it has the same shape and appearance as the physical body (defined as the sthula sarira or "gross body" in Theosophy). The body conforms to the shape or condition of this double.
Blavatsky distinguishes between the linga sarira or astral double and the mayavi-rupa. The linga sarira can be separated or projected only within a limited distance from the body. At death, it is discarded together with the physical body by the inner principles of a human being, and eventually disintegrates or decomposes. The mayavi-rupa in contrast is not vehicle, but an illusory body constructed by kriya-sakti, or the power to create. Apparitions of the dead are often projections of the mayavi-rupa. The latter is projected involuntarily by the dying person.
When the linga sarira is separated from the body, it can be wounded by sharp objects. When it returns to the physical frame, the wound will be reflected in the physical counterpart. This phenomenon is called "repercussion."
Adyar Theosophy and New Age thought
Annie Besant in The Ancient Wisdom writes that the Linga Sarira corresponds to the Etheric Double on the Physical plane. This has been criticized as contrary to orthodox theosophical teachings by Geoffrey Farthing  and others. In the writings of C.W. Leadbeater the astral body is considered a supra-physical reality, equivalent to the kama principle of Blavatsky's septenary series.
In Neo-Theosophical and hence some New Age thought (inspired by both Blavatskyian and Adyar/Neo-Theosophy and Eastern philosophy in general), the Linga Sarira is considered the vehicle of prana (Blavatsky saw Prana and the Linga Sarira as two distinct principles) or qi, and this identified with the Vedantic pranamayakosa (body or "sheath" made of prana). It is also associated with the human aura observed through Kirlian photography or Kilner screens).