Key message: The definition of health must be understandable and motivating.
The World Health Organisation (WHO) defined 1948:
"Health is a state of complete physical, mental and social well being and not merely the absence of disease or infirmity." (www.who.int/about/who-we-are/constitution, 20.06.20)
This definition does not meet the requirements for a definition (read on Learn-Study-Work "How to define words (terms)").
"A definition is the explanation of a previously unknown word with a combination of at least two previously known words." (Seffert , H. (1997) Einführung in die Wissenschaft. München: Beck Verlag, page 49)
I have two problems with the WHO definition:
Conclusion: The definition of health must be easy to understand and motivating.
Definition: Health is the state of a living organism when it functions and feels normal or better than normal.
When a living organism functions and feels normal it is in a state of wellbeeing. This is easy to understand. Example: If a bird cannot fly, then it is sick. If a human being can't fly, that is normal.
This definition motivates me to try to get into a state that is better than normal.
"The concept of wellbeing comprises two main elements: feeling good and functioning well." (www.lifewideeducation.uk/uploads/1/3/5/4/13542890/a4._norman_jackson.pdf, 20.06.20, page 1)
"Body functions are the physiological or psychological functions of body systems. ... No part of the body, from the smallest cell to a complete body system*, works in isolation. All function together, in fine-tuned balance, for the well being of the individual and to maintain life.
*The different systems within the body are: 1. Cardiovascular system 2. Digestive system 3. Endocrine system 4. Integumentary system 5. Lymphatic system 6. Muscular system 7. Nervous system 8. Reproductive system 9. Respiratory system 10. Skeletal system 11. Urinary system " (https://training.seer.cancer.gov/anatomy/body/functions.html , 08.10.21)
"For Aristotle, living a good life meant achieving one’s potential in knowledge, health, friendship, wealth, and other life domains ... Two conceptual approaches to wellbeing research now dominate the field ... The objective approach defines wellbeing in terms of quality of life indicators such as material resources (e.g. income, food, housing) and social attributes (education, health, political voice, social networks and connections). The subjective approach emphasises subjective wellbeing, that is people’s own evaluations of their lives, especially their life satisfaction (a cognitive evaluation), happiness (a positive emotional state) and unhappiness (a negative emotional state)" (www.ncbi.nlm.nih.gov/pmc/articles/PMC5047468/pdf/pone.0163345.pdf, 20.06.20, page 1-2)
"Measures of subjective well being ask individuals to self report ratings of aspects of their lives, including satisfaction with their live as a whole, their feelings at a particular moment, or the extent to which their life have meaning or purpose. These measures focus on what people believe and report feeling, not their objective conditions, although they can be related to objective conditions." (For Good Measure Advancing Research on Well-being Metrics Beyond GDP, DOI: https ://doi.org/10.1787/9789264307278 en, 20.06.20, page 164)
Conclusion: wellbeing = subjective well-feeling + well-functioning
Human beings function and feel better when they follow a healthy lifestyle.
Read on Learn-Study-Work: What is Occupational Health?