Pain is a sensory and emotional experience that typically consists of unpleasantness and aversion. It is a feeling common, for instance, to a headache and a stubbed toe. It has location, duration, intensity and a distinctive quality (e.g., burning, stabbing, dull). The International Association for the Study of Pain (IASP) defines pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage".
This often quoted definition was first published in 1979 by IASP in 'Vol 6 of the journal Pain, 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. A definition reflecting the subjective nature of pain, and widely used in nursing, was introduced by Margo McCaffery in 1968: "Pain is whatever the experiencing person says it is, existing whenever he says it does".
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