General terms

There is no clear and consistent definition as to the exact nature of alternative or complementary medicines. In a 2005 report entitled Complementary and Alternative Medicine in the United States the Institute of Medicine (IOM) adopted this definition:

"Complementary and Alternative Medicine (CAM) is a broad domain of resources that encompasses health systems, modalities, and practices and their accompanying theories and beliefs, other than those intrinsic to the dominant health system of a particular society or culture in a given historical period. CAM includes such resources perceived by their users as associated with positive health outcomes. Boundaries within CAM and between the CAM domain and the domain of the dominant system are not always sharp or fixed."

Other groups and individuals have offered various definitions and distinguishing characteristics. The National Center for Complementary and Alternative Medicine (NCCAM) defines CAM as "a group of diverse medical and health care systems, practices, and products, that are not currently part of conventional medicine." NCCAM has developed what the IOM calls "[o]ne of the most widely used classification structures" for the branches of complementary and alternative medicine. The Cochrane Complementary Medicine Field says:

"What are considered complementary or alternative practices in one country may be considered conventional medical practices in another. Therefore, our definition is broad and general: complementary medicine includes all such practices and ideas which are outside the domain of conventional medicine in several countries and defined by its users as preventing or treating illness, or promoting health and well-being. These practices complement mainstream medicine by 1) contributing to a common whole, 2) satisfying a demand not met by conventional practices, and 3) diversifying the conceptual framework of medicine."

David M. Eisenberg defines it as "medical interventions not taught widely at US medical schools or generally available at US. hospitals," while Richard Dawkins sardonically defines it as a "set of practices which cannot be tested, refuse to be tested, or consistently fail tests."

The term "alternative medicine" is generally used to describe practices used independently or in place of conventional medicine. The term "complementary medicine" is primarily used to describe practices used in conjunction with or to complement conventional medical treatments. NCCAM suggests "using aromatherapy therapy in which the scent of essential oils from flowers, herbs, and trees is inhaled in an attempt to promote health and well-being and to help lessen a patient's discomfort following surgery" as an example of complementary medicine. The terms "integrative" or "integrated medicine" indicate combinations of conventional and alternative medical treatments which have some scientific proof of efficacy; such practices are viewed by advocates as the best examples of complementary medicine. Ralph Snyderman and Andrew Weil go so far as to claim that "integrative medicine is not synonymous with complementary and alternative medicine. It has a far larger meaning and mission in that it calls for restoration of the focus of medicine on health and healing and emphasizes the centrality of the patient-physician relationship." The combination of orthodox and complementary medicine with an emphasis on prevention and lifestyle changes is known as Integrated health.