Chakra is a part of esoteric medieval era theories about physiology and psychic centers that emerged across Indian traditions. The theory posited that human life simultaneously exists in two parallel dimensions, one “physical body” (sthula sarira) and other “psychological, emotional, mind, non-physical” it called the “subtle body” (suksma sarira). This subtle body is energy, while the physical body is mass.
The psyche or mind plane corresponds to and interacts with the body plane, and the theory posits that the body and the mind mutually affect each other. The subtle body consists of nadi (energy channels) connected by nodes of psychic energy it called chakra. The theory grew into extensive elaboration, with some suggesting 88,000 chakras throughout the subtle body. The chakra it considered most important varied between various traditions, but they typically ranged between four and seven.
The important chakras are stated in Buddhist and Hindu texts to be arranged in a column along the spinal cord, from its base to the top of the head, connected by vertical channels. The tantric traditions sought to master them, awaken and energize them through various breathing exercises or with assistance of a teacher. These chakras were also symbolically mapped to specific human physiological capacity, seed syllables (bija), sounds, subtle elements (tanmatra), in some cases deities, colors and other motifs.
The chakra theories of Buddhism and Hinduism differs from the historic Chinese system of meridians in acupuncture. Unlike the latter, the chakra relates to subtle body, wherein it has a position but no definite nervous node or precise physical connection. The tantric systems envision it as a continually present, highly relevant and a means to psychic and emotional energy. It is useful in a type of yogic rituals and meditative discovery of radiant inner energy (prana flows) and mind-body connections. The meditation is aided by extensive symbology, mantras, diagrams, models (deity and mandala). The practitioner proceeds step by step from perceptible models, to increasingly abstract models where deity and external mandala are abandoned, inner self and internal mandalas are awakened.
These ideas are not unique to Buddhist and Hindu traditions. Similar and overlapping concepts emerged in other cultures in the East and the West, and these are variously called by other names such as subtle body, spirit body, esoteric anatomy, sidereal body and etheric body.
According to Geoffrey Samuel and Jay Johnston, professors of Religious studies known for their studies on Yoga and esoteric traditions:
Ideas and practices involving so-called ‘subtle bodies’ have existed for many centuries in many parts of the world. (…) Virtually all human cultures known to us have some kind of concept of mind, spirit or soul as distinct from the physical body, if only to explain experiences such as sleep and dreaming. (…) An important subset of subtle-body practices, found particularly in Indian and Tibetan Tantric traditions, and in similar Chinese practices, involves the idea of an internal ‘subtle physiology’ of the body (or rather of the body-mind complex) made up of channels through which substances of some kind flow, and points of intersection at which these channels come together. In the Indian tradition the channels are known as nadi and the points of intersection as cakra.— Geoffrey Samuel and Jay Johnston, Religion and the Subtle Body in Asia and the West: Between Mind and Body