A recognized relationship between part or parts of different sectors of society which has been formed to take action on an issue to achieve health outcomes or intermediate health outcomes in a way which is more effective, efficient or sustainable than might be achieved by the health sector acting alone. For practical purposes, intersectoral action and multisectoral action are synonymous terms, the former perhaps emphasizing the element of coordination, the latter the contribution of a number of sectors. 
WHO Definitions of Intersectoral Action
WHO Conference on Intersectoral Action for Health 
A recognized relationship between part or parts of the health sector with part or parts of another sector which has been formed to take action on an issue to achieve health outcomes (or intermediate health outcomes) in a way that is more effective, efficient or sustainable than could be achieved by the health sector acting alone.
WHO Health equity through intersectoral action 
Intersectoral action refers to actions affecting health outcomes undertaken by sectors outside the health sector, possibly, but not necessarily, in collaboration with the health sector.
WHO Ageing & Health Glossary 
intersectoral action / multisectoral action: A recognized relationship between part or parts of different sectors of society which has been formed to take action on an issue to achieve health outcomes or intermediate health outcomes in a way which is more effective, efficient or sustainable than might be achieved by the health sector acting alone.  For practical purposes, intersectoral action and multisectoral action are synonymous terms, the former perhaps emphasizing the element of coordination, the latter the contribution of a number of sectors.
Other Definition of Intersectoral Action
Robert Wood Johnson Foundation and Urban Institute 
"The third of the five overarching challenges discussed by the European Commission refers to developing broad social commitment to reduce health inequalities. This is described as working with different levels of government and across sectors (health care, employment, social protection, environment, education, youth, and regional development), to influence how people live their lives, including their experiences at work, school, and leisure in their communities."
Additional Notes on Intersectoral Action
Freiler et al. (2013) 
The concept of intersectoral action developed in several waves, preceding that of HiAP (Health in All Policies).  In 1978, the Alma-Ata Declaration on Primary Health Care called for action by health, social and economic sectors for the ‘attainment of the highest possible level of health’, leading to the Health for All movement in the 1980s and 1990s.  An early definition originated at the 1997 World Health Organization’s Conference on Intersectoral Action for Health: A recognised relationship between part or parts of the health sector with part or parts of another sector which has been formed to take action on an issue to achieve health outcomes (or intermediate health outcomes) in a way that is more effective, efficient or sustainable than could be achieved by the health sector acting alone.  Intersectoral action may occur across various levels of government and between governmental and non-governmental sector, and does not necessarily rely on formal structures (ie, committees, legislation).  As a result, they may be ad hoc in nature, unlike the HiAP approach whose design is for long-term endurance. Intersectoral action may be issue-specific (eg, tobacco control) or centred around responding to systemic concerns (eg, overall quality of life).  During HiAP implementation, individual projects may be issue-specific but a HiAP mandate focuses distinctly on addressing systemic considerations (see intersectoral model in Solar et al). 
Therefore, if intersectoral action is the coordination of various sectors towards the improvement of health equity, HiAP should be considered the most administratively integrated, formal and systemically-focused form of intersectoral action. Intersectoral action may also be referred to as intersectoral initiatives, intersectoral approach or whole-of-government approach. However, whole-of-government approach implies a horizontal, government-only arrangement, whereas intersectoral action can include both horizontal and vertical relationships as well as non-governmental agencies. 
Littlejohns & Smith (2014) 
*Case of 'intersectoral collaborations' to address 'social determinants of health' in Six Western Canada municipalities
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