Reiki is said by its practitioners to be a form of complementary or alternative medicine, developed (or rediscovered) during the Meiji period (the late 19th century) by Mikao Usui (usui mikao) in Japan. It has gained popularity throughout the Western World. Nevertheless, because of the lack of objective evidence for its theories or its results, the scientific establishment considers Reiki to be quackery.

The name Reiki comes from two Japanese characters that describe the energy itself. 'rei' (meaning 'spirit') and ki (meaning 'life force energy' in this context). Common translations of the term reiki are "aura" and also "universal life force energy."

The Japanese noun "Reiki" has been adopted into English and adapted for use as a verb or adjective. In Japan, however, the term is commonly understood as describing a generic term for "ghostly power" and is not interchangeable with the "Usui Method of Reiki Healing" (in Japanese - "usui reiki shiki ryoho").

Practitioners claim that they are able to act as channels for "Reiki" energy, which they allege flows from the universe through their bodies and primarily from the palms of their hands, to specific parts of their own or others' bodies in order to facilitate healing. Some claim that Reiki energy can also be channelled through other parts of a practitioner's body (for example their feet, fingers, or through their eyes) if they wish to. They say the primary method to focus the desired flow of the energy is the practitioner's intention.

Many scientists, health care workers and others dispute the effectiveness of Reiki, claiming that there are no objective studies confirming the existence of this specific Reiki energy or practitioners' claims that this Reiki energy has the capacity to facilitate healing beyond that expected from the placebo effect. At the same time, some healthcare workers (medical doctors, nurses, mental health professionals, hospice and nursing home workers, and other healthcare providers) believe that Reiki has some beneficial effect on the recipient and is a worthwhile inclusion in both professional training and patient care (to wit, registered nurses may earn continuing education units, or CEUs, through the American Holistic Nurses Association, accredited by the American Nurses Credentialing Center, for Reiki training.)