Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage." Postoperative pain is an unpleasant sensory, emotional and mental experience, that is precipitated by surgical trauma and is linked with various autonomic, endocrine–metabolic, physiological and behavioral responses