Health is the functional and/or metabolic efficiency of an organism, at any moment in time, at both the cellular and global levels. In any organism, health is the ability to efficiently respond to challenges (stressors) and effectively restore and sustain a "state of balance," known as homeostasis. Sickness is merely the absence of health. All organisms, from the simplest to the most complex, reside on a spectrum between 100% health and 0% health.

One widely accepted definition of health is that of the World Health Organization "WHO". It states that "health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity" (WHO, 1946). In more recent years, this statement has been modified to include the ability to lead a "socially and economically productive life." The WHO definition is not without criticism, as some argue that health cannot be defined as a state at all, but must be seen as a process of continuous adjustment to the changing demands of living and of the changing meanings we give to life. The WHO definition is therefore considered by many as an idealistic goal rather than a realistic proposition.

The LaLonde report suggested that there are four general determinants of health which he called "human biology", "environment", "lifestyle", and "healthcare organization"[1] Thus, health is maintained through the science and practice of medicine, but can also be improved by individual effort. Physical fitness, weight loss, healthy eating, stress management training and stopping smoking and other substance abuse are examples of steps to improve one's health. Workplace programs are recognized by an increasingly large number of companies for their value in improving health and well-being of their employees, and increasing morale, loyalty and productivity at work. A company may provide a gym with exercise equipment, start smoking cessation programs, provide nutrition, weight or stress management training. Other programs may include health risk assessments, health screenings and body mass index monitoring.

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

Wellness is a term sometimes used to describe the psychological state of being healthy, but is most often used in the field of alternative medicine to describe one's state of being.