^This often quoted definition was first published in 1979 by IASP in Pain journal, number 6, page 250. It is derived from a definition of pain given earlier by Harold Merskey: "An unpleasant experience that we primarily associate with tissue damage or describe in terms of tissue damage or both." Merskey, H. (1964), An Investigation of Pain in Psychological Illness, DM Thesis, Oxford.
Pain. An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Note: The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment. Pain is always subjective. Each individual learns the application of the word through experiences related to injury in early life. Biologists recognize that those stimuli which cause pain are liable to damage tissue. Accordingly, pain is that experience we associate with actual or potential tissue damage. It is unquestionably a sensation in a part or parts of the body, but it is also always unpleasant and therefore also an emotional experience. Experiences which resemble pain but are not unpleasant, e.g., pricking, should not be called pain. Unpleasant abnormal experiences (dysesthesias) may also be pain but are not necessarily so because, subjectively, they may not have the usual sensory qualities of pain. Many people report pain in the absence of tissue damage or any likely pathophysiological cause; usually this happens for psychological reasons. There is usually no way to distinguish their experience from that due to tissue damage if we take the subjective report. If they regard their experience as pain and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain. This definition avoids tying pain to the stimulus. Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause.
^McCaffery M. Nursing practice theories related to cognition, bodily pain, and man-environment interactions. LosAngeles: UCLA Students Store. 1968.
^More recently, McCaffery defined pain as "whatever the experiencing person says it is, existing whenever the experiencing person says it does.” Pasero, Chris; McCaffery, Margo. Pain: clinical manual. St. Louis: Mosby. 1999. ISBN 0-8151-5609-X..
^From the American Board of Medical Specialties website: "Pain Medicine is the medical discipline concerned with the diagnosis and treatment of the entire range of painful disorders. (...) Due to the vast scope of the field, Pain Medicine is a multidisciplinary subspecialty (...)."
^"Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause." Source: IASP Pain Terminology.
^Compare definitions at IASP Pain Terminology: "Neurophathic pain — Pain initiated or caused by a primary lesion or dysfunction in the nervous system." and "Neurogenic pain — Pain initiated or caused by a primary lesion, dysfunction, or transitory perturbation in the peripheral or central nervous system."