The word health means different things to different people, depending on the situation. If somebody says "I was worried about my husband's health when he climbed Mt. Everest", it is clear that the woman is referring to her husband's physical health, possibly his heart, skin (frostbite) and risk of developing hypothermia (when the body's temperature drops too low.
On the other hand, if you hear the phrase "With all these deadlines, presentations and working weekends, I wonder what the effect will be on her health," most likely the word "health" refers more to mental health than physical health (although the two are often linked).
The words "health" or "healthy" can also be used in non-medical contexts. For example "A healthy economy needs an ideal GDP growth rate that is sustainable, one that remains in the expansion phase of the business cycle as long as possible."
The English word "health" comes from the Old English word hale, meaning "wholeness, being whole, sound or well,". Hale comes from the Proto-Indo-European root kailo, meaning "whole, uninjured, of good omen". Kailo comes from the Proto-Germanic root khalbas, meaning "something divided".
The most famous modern definition of health was created during a Preamble to the Constitution of the World Health Organization as adopted by the International Health Conference, New York, 19-22 June, 1946; signed on 22 July 1946 by the representatives of 61 States (Official Records of the World Health Organization, no. 2, p. 100) and entered into force on 7 April 1948.
"Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity."
The Definition has not been amended since 1948.
During the Ottawa Charter for Health Promotion in 1986, the WHO said that health is:
"a resource for everyday life, not the objective of living. Health is a positive concept emphasizing social and personal resources, as well as physical capacities."
An article in The Lancet states that health is not a "state of complete physical, mental, and social well-being". Neither is it "merely the absence of disease or infirmity". The article says the WHO definitions of health will not do in an era marked by new understandings of disease at molecular, individual, and societal levels. (The Lancet, Volume 373, Issue 9666, Page 781, 7 March 2009).
Most people accept that health can be divided into two broad aspects - physical and mental health.

For humans, physical health means a good body health, which is healthy because of regular physical activity (exercise), good nutrition, and adequate rest.
As a country's or region's people experience improved nutrition, health care, standards of living and quality of life, their height and weight generally increase.
In fact, most people, when asked for a definition of health talk about physical health. Physical health relates to anything concerning our bodies as physical entities. Physical health has been the basis for active living campaigns and the many nutrition drives that have swept the industrialized world. People are exposed to so much "physical health" data these days that it is hard to decide what is relevant and what is not.
Another term for physical health is physical wellbeing. Physical wellbeing is defined as something a person can achieve by developing all health-related components of his/her lifestyle. Fitness reflects a person's cardiorespiratory endurance, muscular strength, flexibility, and body composition. Other contributors to physical wellbeing may include proper nutrition, bodyweight management, abstaining from drug abuse, avoiding alcohol abuse, responsible sexual behavior (sexual health), hygiene, and getting the right amount of sleep.
Some people divide physical health into two separate sections:
We may inhale or swallow natural and synthetic chemicals; they can also get into our body through skin. In most cases, the body can break these chemicals down or excrete them, so that there is no risk of toxic overload.
Some chemicals can harm or destroy cells and tissues, while others may affect genetic material directly, which can increase the risk of developing cancer.
Mental health refers to people's cognitive and emotional well-being.

A person who enjoys good mental health does not have a mental disorder. According to WHO, mental health is "a state of well-being in which the individual realizes his or her own abilities, can cope with the normal stresses of life, can work productively and fruitfully, and is able to make a contribution to his or her community".
No matter how many definitions people try to come up with regarding mental health, its assessment is still a subjective one.
People have always found it easier to explain what mental illness is, rather than mental health. Most people agree that mental health refers to the "absence of mental illness". For some, this definition is not enough. They argue that if you pick 100 people who do not suffer from any mental disorder or illness that could be diagnosed by a psychiatrist, some people within those 100 will be mentally healthier than others. Most people also agree that mental health includes the ability to enjoy life, the ability to bounce back from adversity, the ability to achieve balance (moderation), the ability to be flexible and adapt, the ability to feel safe and secure, and self-actualization (making the best of what you have).