Is a concept that functions at the intersection of ethics, law, and politics. It is concerned with the appropriate application of rules, norms, and laws. It focuses on the way that decisions are made or the process used to make those decisions. A specific focus is on fair processes and procedures in decision-making. 
It depends on the level of acceptance by various direct and external stakeholders. Representation, inclusiveness, and transparency are critical to building the necessary trust for legitimacy. In addition, it depends on the ability to engage stakeholders in a meaningful dialogue in which they feel ownership and the possibility of deriving benefits, which requires full transparency, openness, and respect. 
Other efinitions of Legitimacy
Bambra et al. (2007) 
In simple terms, legitimacy means rightfulness; the right to be obeyed and to exercise authority.  Many governments’ health-promotion interventions attempt to influence behaviour on the basis of authority and legitimacy.
Definitions of legitimacy most often focus (1) on who exercises it, (2) on what basis they exercise it, and (3) to what end it is exercised.
Additional Notes on Legitimacy
Given these considerations, the approach to legitimacy presented here departs from two basic assumptions:
- First, every political order designed to last in time engages in the strategic procurement of legitimacy. The operations carried out by rulers to legitimise a political order shape the process and outcome of political decision-making as well as the implementation of public policies. From the perspective of those who stage these operations (the rulers), legitimation is successful to the degree that it allows the regime to effectively guide the political behaviour of the members of society.
- Second, this procurement of legitimacy is dialogical by nature: At the end of the legitimacy chain, it is the individual member of society (the ‘citizen’ in republican terms) who provides legitimacy – even though political collectivities (parties, trade unions, business associations, etc.) often act as vehicles, amplifiers or filters. Citizens respond to the legitimacy claims of rulers by either endorsing or rejecting these claims. At the same time, they express legitimation demands – expectations directed towards their governments, which rulers can decide to meet, repress or compensate.6 From the perspective of political subjects, the success of legitimation lies in the effective common-good orientation of the political regime.
"[Raymand] Aron believed that any such order requires mechanisms that help define the concept of legitimate action—norms, laws, and institutions. But he also recognized that any conception of international order must acknowledge that international actors will always have recourse to force. Ultimately, major states will only obey mechanisms that recognize their rights to sovereignty and self-defense—according to their own definition."
Definitions of organisational legitimacy are relatively broad, and tend toward vague assertions about legitimation arising from consistency with socio-cultural values. Frequently cited definitions of the term include:
'Legitimacy is a generalised perception or assumption that the actions of an entity are desirable, proper, or appropriate within some socially constructed system of norms, values, beliefs, and definitions' (Suchman, 1995: 574).
'Organisational legitimacy refers to the degree of cultural support for an organisation - the extent to which the array of established cultural accounts provide explanations for its existence, functioning, and jurisdiction …' (Meyer and Scott, 1983: 201).
Legitimate organisations meet and conform to societal expectations, and as a result are accepted, valued and taken for granted as right, fitting and good (Aldrich and Fiol, 1994; Meyer and Scott, 1983).
Note: This definition is tied to organizational legitimacy.
Hanberger (2003) 
"Generally, public policy could contribute to recreating legitimacy for an existing or evolving order, hence it is important to consider a policy's history, structural changes and possible futures...Attention will be paid to how local problems and policies have evolved and whether, and to what extent, problem-oriented and problem-effective local government policies have contributed to legitimacy."
Cases of Legitimacy
- "A bi‐dimensional definition of health policy legitimacy – encompassing both public satisfaction with the health system and the normative expectation as to the extent of state involvement in health care – is adopted." | He, A. J. (2018). Public satisfaction with the health system and popular support for state involvement in an east asian welfare regime: Health policy legitimacy of hong kong. Social Policy and Administration, 52(3), 750-770. doi:http://dx.doi.org/10.1111/spol.12274
- 3 components of legitimacy: substantive legitimacy; procedural legitimacy; feasibility-centered legitimacy | Park, C., Lee, J., & Chung, C. (2015). Is "legitimized" policy always successful? policy legitimacy and cultural policy in korea. Policy Sciences, 48(3), 319-338. doi:http://dx.doi.org/10.1007/s11077-015-9220-2
- "According to Jean-Marc Coicaud and David Ames Curtis, political legitimacy arises from three sources: the law, the population's norms, and the population's consent" | Jean-Marc Coicaud and David Ames Curtis, Legitimacy and Politics: A Contribution to the Study of Political Right and Political Responsibility (Cambridge: Cambridge University Press, 2002).
- Wiechnik, S. J. (2012). Policy, COIN doctrine, and political legitimacy. Military Review, 92(6), 22-30. Retrieved from http://utsph.idm.oclc.org/login?url=https://search.proquest.com/docview/1268177133?accountid=7134
- "the processes of stakeholder engagement shape the legitimacy of public policies and the governments who promote them." | Wallner, J. (2008). Legitimacy and public policy: Seeing beyond effectiveness, efficiency, and performance. Policy Studies Journal, 36(3), 421-443. doi:http://dx.doi.org/10.1111/j.1541-0072.2008.00275.x
 Dawson A. &Verweij M. 2014. Public Health and Legitimacy: or why there is still a place for substantive work in ethics. Public Health Ethics, 7(2): 95-97.
 Kickbusch I &Gleicher D. 2012. Governance for health in the 21st century. WHO-Regional office for Europe. Copenhagen, Denmark.
 Bambra C, Fox D, Scott-Samuel A. A politics of health glossary. Journal of Epidemiology & Community Health. 2007 Jul 1;61(7):571-4.
 Heywood A. Political ideas and concepts. London: Macmillan, 1994.
 Hanberger, A. (2003). Public policy and legitimacy: A historical policy analysis of the interplay of public policy and legitimacy. Policy Sciences, 36(3), 257-278. doi:http://dx.doi.org/10.1023/B:OLIC.0000017471.88620.9a