As defined in the 1946 constitution of the World Health Organization, "health is a state of complete physical, mental, and social well-being, and not merely the absence of disease or infirmity." 

The term mental health represents a variety of human aspirations: 

Rehabilitation of the mentally disturbed, 

Prevention of mental disorder, 

Reduction of tension in a stressful world, 

and Attainment of a state of well-being in which the individual functions at a level consistent with his or her mental potential. 

As noted by the World Federation for Mental Health, the concept of optimum mental health refers not to an absolute or ideal state but to the best possible state in so far as circumstances are alterable. 

Mental health is regarded as a condition of the individual, relative to the capacities and social-environmental context of that person.

Medical and Alternative Medical Terminology, Explanations -  by Courtesy of Encyclopedia Britannica® 2001