Pain is the unpleasant and aversive feeling common to such experiences as a stubbed toe, a headache, a burnt finger, and salt in a wound. Typically, pain is characterized by its intensity, location and duration. 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. Cultural values, hypnotic suggestion, and cognitive activities such as distraction or appraisal can all significantly modulate pain's intensity and unpleasantness. 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". A definition widely employed in nursing, emphasizing the subjective nature of pain and the importance of believing patient reports, was introduced by Margo McCaffery in 1968: "Pain is whatever the experiencing person says it is, existing whenever he says it does". '

Pain is a major symptom in many medical conditions, and can significantly interfere with a person's quality of life and general functioning. It is the single most frequent reason for physician consultation in the United States. Usually pain stops without treatment or responds to simple measures such as resting or taking an analgesic, and it is then called ‘acute’ pain. But it may also become intractable and develop into a condition called chronic pain, in which pain is no longer considered a symptom but an illness by itself.

The study of pain has in recent years attracted many different fields such as pharmacology, neurobiology, nursing, dentistry, physiotherapy, and psychology. Pain medicine is a separate subspecialty figuring under some medical specialties like anesthesiology, physiatry, neurology, and psychiatry.

Physical pain is linked to various cultural, religious, philosophical, or social issues.