You know what pain is. Everyone does. But defining it in words isn’t easy. The word “pain” comes from the Latin word “poena,” meaning penalty or punishment, and unfortunately pain still carries that connotation. Medical definitions tend to be circular, relying on words like “discomfort” or “distress” that essentially just say “pain” in some other way. Researchers often use the definition developed by the International Association for the Study of Pain: “Pain is an unpleasant sensory or emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” John Loeser, a University of Washington physician, has divided pain into four components: detection of damage to human tissue, the brain’s perception of that damage, the emotional response to that perception (which he labels “suffering”), and the behaviors in response to those emotions and perceptions.
For most people, particularly people who are feeling pain, the only definition that really matters is that it hurts. Or as two prominent pain researchers wrote several years ago: Pain is “whatever the experiencing person says it is, existing whenever he [or she] says it does.”

Types of pain. Pain is most often classified according to how long it lasts and what causes it. Using this yardstick, there are three basic types of pain: transient, acute, and chronic.

Transient pain. The everyday bumps, nicks, and little cuts of life cause transient or fleeting pain. One example of transient pain is an occasional, mild headache that’s relieved by over-the-counter drugs. You probably wouldn’t go to a doctor for transient pain unless it was somehow associated with a significant injury.

Acute pain. pain that begins abruptly and has a foreseeable end — is more complicated. It’s the body’s natural response to an adverse chemical, thermal, or mechanical stimulus. Examples include any kind of injury, surgery, or childbirth (see Pain with a purpose). The boundary between acute pain and long-lasting or chronic pain is sometimes blurry. Pain experts agree that acute pain associated with tissue injury can persist anywhere from less than one month to more than six months. However, any pain that continues for more than a few months causes physical and emotional changes that muddy the distinction between acute and chronic. Moreover, even brief periods of acute pain are capable of causing the kind of long-term changes to nerve cells that can permanently excite them and lead to chronic pain.

Chronic pain. This type of pain can start with an injury or disease but persists well after the injury is healed or the disease is cured or goes away. Some pain experts prefer the term “persistent pain.” One prominent pain researcher wrote that with chronic pain, “it is almost as if the brain develops a memory for pain, much like the skill of learning to ride a bicycle is never unlearned.” Cancer, arthritis, diabetes, and a variety of other conditions are frequent sources of chronic pain. The presence of chronic pain is a key part of the definition of certain ailments, such as low back pain, some forms of headaches, and fibromyalgia. Many types of chronic pain stem partly or entirely from damage to the nerves themselves — a condition known as neuropathic or nerve pain. In particular, the pain feelings associated with certain types of diabetes, shingles, and AIDS are characteristic of nerve pain