Pain is the unpleasant and aversive feeling common to such experiences as stubbing a toe, burning a finger, putting iodine on a cut and bumping the "funny bone". The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage".

Pain motivates us to withdraw from damaging or potentially damaging situations, protect the damaged body part while it heals, and avoid those situations in the future. It is initiated by stimulation of nociceptors in the peripheral nervous system, or by damage to or malfunction of the peripheral or central nervous systems. Most pain resolves promptly once the painful stimulus is removed and the body has healed, but sometimes pain persists despite removal of the stimulus and apparent healing of the body; and sometimes pain arises in the absence of any detectable stimulus, damage or pathology. Cultural values, hypnotic suggestion, and cognitive activities such as distraction or appraisal can all significantly modulate pain's intensity and unpleasantness.

Pain is the single most frequent reason for physician consultation in the United States. It is a major symptom in many medical conditions, and can significantly interfere with a person's quality of life and general functioning. Pain medicine is a subspecialty under such medical specialties as anesthesiology, physiatry, neurology, and psychiatry. The study of pain has in recent years attracted many different fields including pharmacology, neurobiology, nursing, dentistry, physiotherapy, and psychology.