The American Heritage® Stedman's Medical Dictionary defines health as;
health; pronunciation: 'helth: noun

"1: the condition of an organism or one of its parts in which it performs its vital functions normally or properly; the state of being sound in body or mind; especially : freedom from physical disease and pain. 2: the condition of an organism with respect to the performance of its vital functions especially as evaluated subjectively or nonprofessionally."

In any organism, health is a form of homeostasis. This is a state of balance, with inputs and outputs of energy and matter in equilibrium (allowing for growth). Health also implies good prospects for continued survival. In sentient creatures such as humans, health is a broader concept. The World Health Organization defines health as "a state of complete physical, mental and social well-being, and does not consist only of the absence of disease or infirmity."

The most solid aspects of wellness that fit firmly in the realm of medicine are the environmental health, nutrition, disease prevention, and public health matters that can be investigated and assist in measuring well-being.

In some societies, health involves managing the body state after the more basic needs of food, shelter and basic medical care have already been met. Many of the practices applied in the pursuit of wellness, in fact, are aimed at controlling the side effects of affluence, such as obesity and lack of exercise.

Wellness grew as a popular concept in the West starting in the late 19th century, just as the middle class began emerging in the industrialized world, and a time when a newly prosperous public had the time and the resources to pursue wellness and other forms of self-improvement. Many early consumer products, from corn flakes to mouth wash, derived from or exploited the emerging interest in wellness.
Wellness can include using scientifically-based tests and practices to maintain health, as in checking cholesterol, blood pressure, glucose, and other body indicators. Or it can include controversial practices, such as avoiding certain foods or taking certain vitamins or alternative medicines.

The subjective nature of "wellness" can be illustrated by the hypothetical example of an individual who avoids food additives and is selective in choosing foods to prolong health, but thinks nothing of getting in a car and driving hundreds of miles. Statistically, the known risk of mortality or morbidity from automobile usage is far greater than the risk of mortality or morbidity from food additives, but avoiding certain foods and food additives feels "healthy," whereas avoiding automobile use feels merely inconvenient.

Even when the techniques used are not scientifically proven, the pursuit of wellness can enhance health by a placebo effect. Someone who feels "well" may lower stress and enhance their sense of well-being, achieving an enhanced psychological state with proven beneficial effects on various body systems, including blood pressure, gastrointestinal system functioning, and immune response. The field of psychoneuroimmunology explores these linkages in a scientific manner, and is also a part of medicine proper. However it is new, and still exploring the biology, and has little or no clear advice to offer other than to avoid unnecessary stress or that which is out of one's control or capacity.