Health is the level of functional and metabolic efficiency of a living organism. In humans it is the ability of individuals or communities to adapt and self-manage when facing physical, mental, psychological and social changes with environment. The World Health Organization (WHO) defined health in its broader sense in its 1948 constitution as “a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.” This definition has been subject to controversy, in particular as lacking operational value, the ambiguity in developing cohesive health strategies, and because of the problem created by use of the word “complete”. Other definitions have been proposed, among which a recent definition that correlates health and personal satisfaction.  Classification systems such as the WHO Family of International Classifications, including the International Classification of Functioning, Disability and Health (ICF) and the International Classification of Diseases (ICD), are commonly used to define and measure the components of health.
The definition of health has evolved over time. In keeping with the biomedical perspective, early definitions of health focused on the theme of the body’s ability to function; health was seen as a state of normal function that could be disrupted from time to time by disease. An example of such a definition of health is: “a state characterized by anatomic, physiologic, and psychological integrity; ability to perform personally valued family, work, and community roles; ability to deal with physical, biological, psychological, and social stress”. Then, in 1948, in a radical departure from previous definitions, the World Health Organization (WHO) proposed a definition that aimed higher: linking health to well-being, in terms of “physical, mental, and social well-being, and not merely the absence of disease and infirmity”. Although this definition was welcomed by some as being innovative, it was also criticized as being vague, excessively broad, and was not construed as measurable. For a long time it was set aside as an impractical ideal and most discussions of health returned to the practicality of the biomedical model.