Pain is an unpleasant sensation which may be associated with actual or potential tissue damage and which may have physical and emotional components. According to the International Association for the Study of Pain (IASP), one should distinguish between pain and nociceptionIASP pain glossary. The word "pain" comes from the Latin "poena" meaning a fine, a penalty.

The term "pain" is a subjective experience that typically accompanies nociception, but can also arise without any stimulus, and thus includes the emotional response. Nociception, on the other hand, is a neurophysiological term and denotes specific activity in nerve pathways. It is the transmission mechanism for physiological pain, and does not describe psychological pain. These pathways transmit the nominally "painful" signals, though they are not always perceived as painful. Although pain can be associated with tissue damage or inflammation, this is often not the case.

Despite its unpleasantness, pain is a critical component of the body's defense system. It is part of a rapid warning relay instructing the motor neurons of the central nervous system to minimize detected physical harm. Lack of the ability to experience pain, as in the rare condition Congenital insensitivity to pain or Congenital Analgesia, can cause many various health problems.