Pain

An unpleasant sensation resulting from damage to nerve tissue, stimulation of free nerve endings, or excessive stimulation (e.g., extremely loud sounds). Physical pain is elicited by stimulation of pain receptors, which occur in groups of myelinated or unmyelinated fibers throughout the body but particularly in surface tissues.

It may also be a feeling of severe distress and suffering resulting from acute anxiety, loss of a loved one, or other psychological factors. Because of these various factors, as well as previous experience in pain response, individual reactions vary widely.

Adapted from the APA Dictionary of Psychology