It is difficult to define complementary and alternative medicine, as it covers a very wide and constantly evolving field. The National Center for Complementary and Alternative Medicine (NCCAM) defines complementary and alternative medicine as a set of systems, practices, and products that are generally not considered part of conventional medicine. Conventional medicine (also called Western or allopathic medicine) is known as medicine practiced by those who possess M.D. (doctor of medicine) or D.O. (doctor of osteopathy) degrees, and associated health professionals such as physiotherapists, psychologists, and registered nurses. The boundaries separating complementary and alternative medicine from conventional medicine are not absolute and, over time, certain practices of complementary and alternative medicine may become generally accepted. “Complementary medicine” refers to the use of complementary and alternative medicine alongside conventional medicine, such as the use of acupuncture in addition to the usual methods of pain relief; another example, massage and guided imagination can be used as a supplement of analgesics to help reduce pain. Most people who use complementary and alternative medicine in the United States do so in a complementary manner.

While the distinction between conventional and alternative medicine is not always easy to determine, there is a basic philosophical difference. Conventional medicine generally defines health as the absence of disease. The main causes of illness are almost always considered as isolated factors, such as bacteria or viruses, biochemical imbalances and aging, and treatment usually includes drugs or surgery. In contrast, alternative medicine usually defines health holistically, i.e. as a balance between the physical, emotional and spiritual systems, which involves the person as a whole. The alteration of the balance between these systems is believed to be the cause of the disease. Treatment involves strengthening the body’s own defenses and restoring this balance.

Complementary and alternative medicine practices are often grouped into a wide range of categories, such as natural products, mind-body medicine, and body-based and manipulative practices. While these categories are not formally defined, they are useful for describing complementary and alternative medicine practices. Some complementary and alternative medicine practices may fall into more than one category.