Ideologies are fundamental beliefs which control, coordinate and influence to the point of subjugation other social basic beliefs. As per Cambridge Advanced Learner's Dictionary online the word ideology is enumerated or defined as noun [C or U] A theory, or set of beliefs or principles, especially one on which a political system, party or organization is based. For example, a religious ideology will control rituals and tolerance capacity toward other religions, feminist ideology will control aspects in gender equity and so on. Ideology "emerges from a context of thought which foregrounds the confrontation between idealism and materialism, from Marx's emphasis upon the material aspects of social life and a parallel suspicion of mental or ideational dimensions" (Purvis & Hunt).
Ideology can also be described as fundamental social values or beliefs which exist in an abstract manner, like for example, equality. Ideologies can also be totally rationalistic and at the same time ideologies can be based on complete irrationality.
As Eagleton expresses it, "An interesting feature of this 'end-of-ideology' ideology is that it tends to view ideology in two quite contradictory ways, as at once blindly irrational and excessively rationalistic." (Eagleton)
What is common to most definitions of ideology is that it 'serves to sustain relations of domination'. This throws up the question whether only those beliefs non-ideological when they are not in power, and ideological when they are This definitely needs a slightly broader accentuation. As Martin Seliger puts it, ideology is a justification for 'organized social action and specifically political action, irrespective whether such action aims to preserve, amend  rebuild a social order'(Eagleton). In the same lines the definition of 'power' changes into a 'intangible network of force which weaves itself into our slightest gestures and most intimate utterances' (Eagleton). In this case, ideology is bound and limited to political power gains, without which the term 'ideology' would cease to exist. It is important to note here that "ideologies are the outcome and function of given relations of production and the class interests they give rise to. They are extensions and instruments of this regime of power - they conceal and 'mask' these relations of domination and the various social contradictions they give rise to" (Purvis & Hunt).
Ideology definition can be further fine-tuned to include the fact that it exists when it can discriminate between power that acts on a respective social order, and the one that is not. Ideologies "are extensions and instruments of this regime of power - they conceal and 'mask' these relations of domination and the various social contradictions they give rise to" (Purvis & Hunt).
Ideology in other words is the means with which men, as social animals, define rules for their socio-political conflicts; the process which uses power to uphold certain social levels; or as Althusser aptly said, "the organization of signifying practices which goes to constitute human being as social subjects, and which produces he lived relations by which such subjects are connected to the dominant relations of production in a society.'(Eagleton).
Ideology is a deifinite set of social beliefs deeply entrenched in the