Health is a unity and harmony within the mind, body and spirit which is unique to each person, and is as defined by that person. The level of wellness or health is, in part, determined by the ability to deal with and defend against stress. Health is determined by physiological, psychological, socio-cultural, spiritual, and developmental stage variables.
Definitions of aging differ between biologists and behavioral scientists. Biologists regard aging as reflecting the sum of multiple and typical biological decrements occurring after sexual maturation; behavioral scientists view it as reflecting regular and expected changes occurring in genetically representative organisms advancing through the life cycle under normal environmental conditions. In humans, aging is associated with degenerative changes in the skin, bones, heart, blood vessels, lungs, nerves, and other organs and tissues. The branch of medicine that deals with the disorders of aging in humans is geriatrics.