Table 1. Features of different health definitions.

Feature	Sources	Commentary
Health is achievement of a common standard.	WHO5	Some define health as the achievement of a defined (aspirational) standard, whilst others describe a more analogue scale whereby health can be achieved to a greater or lesser extent (and possibly with lower expectations given contextual and personal circumstances). For epidemiological study, a common definition that is not context specific can help identify exposures which create limits on the experience of positive health which might otherwise be ignored.
Health is achievement of an ‘ideal’ outcome.	WHO5 Elrick10	The definitions of health which categorise people into healthy or not on the basis of whether they have achieved a ‘complete’ state of health or well-being are good for recognising aspiration and potential. However, they may not recognise that people can see themselves as healthy whilst living with some forms of disability or conditions, and they may not recognise the process of ‘healthy ageing’ whereby some loss of functionality may not represent a loss of health.
Health is experiential.	Card13	The experience of positive or negative health as an experience in and of itself (i.e. separate from the capacity this may provide to function or participate in the economy or society) is not a ubiquitous feature of definitions. Some argue that it is not the experience of health that matters (or indeed that can be defined) but instead the capacities it provides which are important. Clearly, the two are linked, and it is difficult to envisage a scenario whereby negative health is experienced without capacity being reduced. However, this may reduce the human experience to an overly functional or mechanistic phenomenon (or even to reduce health to the ability to be productive in society) and therefore undermine the experience and value of health for its own sake.
Health is the ability to function and participate.	WHO6 Starfield11 Leonardi12 Last20	Some define health solely on the (in)ability to participate in society (otherwise framed as a resource for living or the ability to ‘function’), whilst others include this as an essential component alongside the physical and mental aspects. Defining health narrowly on the basis of participation in society means that experiential elements (pain, low mood, etc.) are only relevant to the extent that they impact on the ability to participate. The advantage of including this aspect is that health is recognised as a contextualised phenomenon in which the extent to which a society enables and includes (for example) people with particular disabilities influences the experience of health.
Health is defined by its determinants.	IUHPE19	Without a definition of the outcome or experience of health, defining health by its determinants alone is imprecise and unsatisfactory. For example, if health is determined by adequate income, all outcomes that are due to adequate income would constitute ‘health’. This would be too broad a definition to be useful. In this way such definitions of health are better covered within a theoretical framework of health causation than in a definition of health.
Health is an individual and population phenomenon.	Starfield11	Some definitions focus only on health as a population phenomenon, but this restricts its applications.
Health is a multidimensional phenomenon.	WHO5 WHO6 Card13	This recognises the holistic nature of the experience of health. Most recent definitions of health recognise the physical and mental components of health and so this is uncontentious.
Health is defined by the control people have over their lives.	WHO6 Scott Samuel17	Health is clearly a resource which determines the control people have over their lives, their ability to realise expectations and to satisfy needs, but it is not the only determining factor (for example, the political and socio-economic context are also very important).
Health has to be sustainable.	Scott Samuel17 Last20	Some definitions of health focus largely, or entirely, on its sustainability. However, this confuses the outcome of interest (health) with the processes through which health is determined.

WHO, World Health Organisation.