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 state 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 medicine.

There is no clear and consistent definition for either alternative or complementary medicine. In Western culture it is often defined as any healing practice "that does not fall within the realm of conventional medicine", or "that which has not been shown consistently to be effective."

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."

The Danish Knowledge and Research Center for Alternative Medicine (Danish abbreviation: ViFAB. ViFAB is an independent institution under the Danish Ministry of the Interior and Health. ViFAB's webstite: www.vifab.dk/uk) uses the term alternative medicine for: Treatments performed by therapists who are not authorized health care professionals. - Treatments performed by authorized health care professionals, but which are based on methods otherwise mainly used outside the health care system. People without a health care authorisation must be able to perform the treatments.

The Cochrane Complementary Medicine Field finds that what is considered complementary or alternative practices in one country may be considered conventional medical practices in another. Their definition is therefore 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."

For example, biofeedback is commonly used within the Physical Medicine & Rehabilitation community, but is considered alternative within the medical community as a whole, and some herbal therapies are mainstream in Europe, but are alternative in the United States. David M. Eisenberg, an integrative medicine researcher, defines it as "medical interventions not taught widely at US medical schools or generally available at US. hospitals," NCCAM states that formerly unproven remedies may be incorporated into conventional medicine if they are shown to be safe and effective.

Barrie R. Cassileth, a researcher of complementary and alternative medicine, has summed up the situation as "not all mainstream physicians are pleased with CAM, with current efforts to integrate CAM into mainstream medicine, or with a separate NIH research entity for "alternative" medicine.

NCCAM has developed one of the most widely used classification systems for the branches of complementary and alternative medicine.It classifies complementary and alternative therapies into five major groups which have some overlap.

1. Whole medical systems: cut across more than one of the other groups; examples include Traditional Chinese medicine, Naturopathy, Homeopathy and Ayurveda.
2. Mind-body medicine: takes a holistic approach to health that explores the interconnection between the mind, body, and spirit. It works under the premise that the mind can affect "bodily functions and symptoms".
3. Biologically based practices: use substances found in nature such as herbs, foods, vitamins, and other natural substances.
4. Manipulative and body-based practices: feature manipulation or movement of body parts, such as is done in chiropractic and osteopathic manipulation.
5. Energy medicine: is a domain that deals with putative and verifiable energy fields:

• Biofield therapies are intended to influence energy fields that purportedly surround and penetrate the body. No empirical evidence has been found to support the existence of the putative energy fields on which these therapies are predicated.
• Bioelectromagnetic-based therapies use verifiable electromagnetic fields, such as pulsed fields, alternating-current or direct-current fields in an unconventional manner.