Health is the practical and metabolic efficiency of an organism, at any moment in time, at both the cellular and global levels. All individual organisms, from the simplest to the most complex, vary among optimum health and zero health.

In the medical field, health is normally defined as an organism's ability to capably respond to challenges and effectively restore and sustain a "state of balance," known as homeostasis.

Another broadly accepted definition of health is that of the World Health Organization "WHO". It states that "health is a state of absolute physical, mental and social well-being and not just the absence of disease or infirmity". In more recent years, this statement has been customized to include the ability to lead a "socially and economically creative life." The WHO definition is not without criticism, as some dispute that health cannot be defined as a state at all, but must be seen as a process of continuous alteration to the changing demands of living and of the changing meanings we give to life. The WHO definition is therefore measured by many as an idealistic goal rather than a realistic intention.

An increasing measure of the health of populations is height, which is powerfully regulated by nutrition and health care, among other average of living and quality of life matters. The study of human growth, its regulators and its implications is known as auxology.