The word legitimacy is often interpreted in a normative or a positive way. In a normative sense, legitimacy gets greater attention as a part of moral philosophy. Legitimacy is the foundation of such governmental power as is exercised both with a consciousness on the government's part that it has a right to govern and with some recognition by the governed of that right.
Something becomes "legitimate" when one approves of it. In a positive sense, legitimacy gets greater attention in political science. For example, an institution is perceived as legitimate, if approval for that institution is general among those people subject to its authority. According to John Locke, the British social contractualist, issues of legitimacy are linked to those of consent, both explicit and tacit.
Legitimacy in political science, is the popular acceptance of a governing regime or law as an authority. Whereas authority refers to a specific position in an established government, the term legitimacy is used when describing a system of government itself—where "government may be generalized to mean the wider "sphere of influence." According to Robert Dahl, legitimacy is considered a basic condition for rule: without at least a minimal amount of legitimacy, a government will lead to frequent deadlocks or collapse in the long run.
Robert A. Dahl has explained the concept of legitimacy by using the metaphor of a reservoir. For example, as long as the reservoir stays at a certain level stability can be maintained, if it falls below the required level it is endangered. Regimes in all the states require the assent of a large proportion of the population in order to retain power. In several countries this is not the case: many unpopular regimes have survived because they are supported and considered as legitimate by a small but influential elite.
In the case of laws, legitimacy should be distinguished from legality. Action can be legal without being legitimate (as in the case of an immoral law). Action can also be legitimate without being legal. When sources of legitimacy clash with one another, constitutional crisis erupts.
Legitimacy as a concept is often applied to other, non-political, kinds of authority, and also to issues concerning the legitimacy of entire political-economic systems (such as capitalism) as discussed in the Marxist tradition.

Types of legitimacy
The dominion of a godking of which ancient Egypt offers the best example, is the theological doctrine according to which every Pharaoh is himself (among other things) the god Horus, son of Osiris. The doctrine seems to go back to the very origin of the empire. The Christian priesthood derived its legitimacy and still does from a source very similar to that of the kingship; according to official doctrine the Papal office is based on Christ's designation of St. Peter, which continues to sanctify and legitimize the rule of every successive pope.

Numinous legitimacy
Civil legitimacy exists when a system of government is based on agreement between equally autonomous constituents who have combined to cooperate towards some common good. Every modern constitutional system or every system of representational government is founded either on a basic agreement to follow certain rules or at least on a justifiable assumption that a basic agreement to follow certain rules exists. Modern constitutional government makes one characteristic of civil legitimacy particularly clear: Governmental offices are ordered by trust rather than exercised by dominion. This is expressed in the institution of public elections.

Civil legitimacy

Sources of legitimacy
The German economist and sociologist Max Weber argued that there are three forms of legitimacy, and that all human societies, across history, have been based on them.
Weber, like the British Philosopher Thomas Hobbes, had an extremely negative and pessimistic view of human nature, and believed that societies often went through cycles. Weber did not see democracy as being necessary for legitimacy, as a government could be legitimized through laws and principles not established by a vote. Weber also claimed that it is perfectly possible for a modern society to revert back and become a follower of a brutal form of charismatic leadership, a phenomenon which later occurred in his home country of Germany under Adolf Hitler and which was also witnessed in other parts of the world, such as Mussolini's Italy.
French political scientist and social thinker Mattei Dogan offers a more contemporary conception of this typology of legitimacy. While Weber's typology (traditional/charismatic/legal-rational) was seminal throughout the previous centuries, Dogan argues that it is insufficient to cover the complex relationships between legitimacy and political systems. In fact, in Dogan's view, the first two types (traditional and charismatic) are today obsolete. The most recent example of charismatic legitimacy dates back to Khomeini. Dogan believes that traditional authority has disappeared completely, with the exception of two or three regimes in the Middle East (like Saudi Arabia). The third type called rational-legal is, in Dogan's view, an amalgamation of many varieties, to such a degree that they no longer constitute a "type."
Old favorite, Rothschild Hornschwagle, offers yet another interpretation of legitimacy in politics. Those who can, do. That which you cannot undo, is legitamite. If you cannot undo what was done before, by any means, Some flavor of legitimacy is conferred upon whatever "that" thing is.

Charismatic authority. Legitimacy based on the charisma of the leader, often partly based on the perception that this leader has certain extra or supernatural attributes. Example: a tribal chieftain or a religious leader.
Traditional authority. Legitimacy based on tradition; e.g., people accept the government for the simple fact that it has been around for so long and is based on popular customs and usages. Example: a monarchy.
Rational/legal authority. Legitimacy based on the perception that a government's powers are derived from set procedures, principles, and laws which are often complex and are written down as part of the constitution. Example: representative democracy or bureaucrats.