A definition of 'hegemony'
This definition is taken from Keywords, by Raymond Williams (pp 145-6). The definition has been slightly edited and emphases in bold have been added.
Hegemony was probably taken directly into English from egemonicz, Gk, egemon, Gk 'leader, ruler', often in the sense of a state other than his own. Its sense of a political predominance, usually of one state over another, is not common before C19, but has since persisted and is now fairly common, together with hegemonic, to describe a policy expressing or aimed at political predominance. More recently, hegemonism has been used to describe specifically 'great power' or 'superpower' politics, intended to dominate others, (indeed hegemonism has some currency as an alternative to IMPERIALISM).
There was an occasional early use in English to indicate predominance of a more general kind. From 1567 there is 'Aegemonie or Sufferaigntie of things growing upon ye earth', and from 1656 'the Supream or Hegemonick part of the Soul'. Hegemonic, especially, continued in this sense of 'predominant' or of a 'master principle'. The word has become important in one form of C20 Marxism, especially from the work of Gramsci (in whose writings, however, the term is both complicated and variable; see Anderson). In its simplest use it extends the notion of political predominance from relations between states to relations between social classes, as in bourgeois hegemony. But the character of this predominance can be seen in a way which produces an extended sense in many ways similar to earlier English uses of hegemonic. That is to say, it is not limited to matters of direct political control but seeks to describe a more general predominance which includes, as one of its key features, a particular way of seeing the world and human nature and relationships.
It is different in this sense from the notion of 'world-view', in that the ways of seeing the world and ourselves and others are not just intellectual but political facts, expressed over a range from institutions to relationships and consciousness. It is also different from IDEOLOGY in that it is seen to depend for its hold not only on its expression of the interests of a ruling class but also on its acceptance as 'normal reality' or 'commonsense' by those in practice subordinated to it. It thus affects thinking about REVOLUTION in that it stresses not only the transfer of political or economic power, but the overthrow of a specific hegemony: that is to say an integral form of class rule which exists not only in political and economic institutions and relationships but also in active forms of experience and consciousness. This can only be done, it is argued, by creating an alternative hegemony - a new predominant practice and consciousness.
The idea is then distinct, for example, from the idea that new institutions and relationships will of themselves create new experience and consciousness. Thus an emphasis on hegemony and the hegemonic has come to include cultural as well as political and economic factors; it is distinct, in this sense, from the alternative idea of an economic base and a political and cultural superstructure, where as the base changes the superstructure is changed, with whatever degree of indirectness or delay. The idea of hegemony, in its wide sense, is then especially important in societies in which electoral politics and public opinion are significant factors, and in which social practice is seen to depend on consent to certain dominant ideas which in fact express the needs of a dominant class.