The International Association for the Study of Pain (IASP) published this definition in 1979:
“PAIN: An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (IASP 1979, p.249).
This definition seems brief, given the obvious complexity of pain. Upon closer inspection, we can appreciate that is does indeed cover a great deal of what is known about pain, and it does so simply and concisely.
It tells us that pain is an unpleasant, multidimensional experience. It recognises that pain can be felt in the absence of tissue damage, yet is described in terms of such damage. It also takes into account that pain is subjective. It is an experience that exists in the mind of the individual in pain.
The subjective and individual nature of pain is explained further by a note which accompanies the definition:
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.
International Association for the Study of Pain. (1979). Pain terms: a list with definitions and notes on usage. Recommended by the IASP Subcommittee on Taxonomy. Pain, 6(3), 249–252.
Merskey & Bogduk 1994, IASP Task Force on Taxonomy Part III: Pain Terms, A Current List with Definitions and Notes on Usage, p.2-3