The word "ideology" was originally coined by Count Destutt de Tracy, a French rationalist philosopher of the late eighteenth century to define a "science of ideas." For de Tracy, ideology formed "a part of zoology" (i.e. biology) The concept of ideology was developed in Marxian thought as a term through which to articulate the relation between the realm of culture and the realm of political economy. For Marx, the proper method for analyzing concepts is one which retraces the steps from the abstract concept back to its concrete origin. 

"If in all ideology men and their relations appear upside-down as in a camera obscura, this phenomenon arises just as much from their historical life-processes as the inversion of objects on the retina does from their physical life-processes." Karl Marx, The German Ideology. (note the analogy between physiology in perception and social life in thought, both function as the concrete origins, if not as determinants.) 

In Marxist theory, every society is crucially defined by its class structure, by the specific relation between the dominant class and the producing or working class, or proletariat. Societies use the apparatuses of ideology to reconcile its social subjects to their structure, using force only when ideology does not suffice. "A society is possible in the last analysis because the individuals carry around in their heads some sort of picture of that society." (Mannheim) -- "and their place in it." (Kavanagh) (see hegemony ) Thus the disavowal of politics and "ideology" in contemporary liberal culture is precisely ideological, whose social function is to obscure the real processes that found one's social life. As Althusser puts it, "Those who are in ideology believe themselves by definition outside ideology...Ideology never says 'I am ideological.'" (Lenin and Philosophy, p.175) 

Althusser (and Lacan) define ideology as "the representation of the subject's imaginary relationship to his or her Real conditions of existence." (Louis Althusser, "Ideological State Apparatuses", in Lenin and Philosophy.) "Ideology offers the social subject a fundamental framework of assumptions that defines the parameters of the real and the self; it constitutes what Althusser calls the social subject's "'lived' relation to the real." (James H. Kavanagh, "Ideology," in Critical Terms for Literary Study) For Althusser the term Ideological State Apparatuses designates the material existence of ideology in ideological practices, rituals, and institutions. 

Ideology is not simply "false consciousness," however, nor can any society dispense with it. 

Henri Lefebvre aks "What is an ideology without a space to which it refers? ...What would remain of the Church if there were no churches?" (p.44) For Lefebvre, "Ideology per se might well be said to consist primarily in a discourseupon social space

For Michel de Certeau, "Denial of the specificity of the place (of production) is the very principle of ideology." ("The Historiographical Operation") "By moving discourse into a non-place, ideology forbids history from speacking of society and of death -- in other words, from being history." (p69) "If religion is one form of ideology, it is also the form of every ideology. Only the religious illusion may found that of a perfect autonomy, characteristic of all ideology." (Sarah Kofman)

Virtuality can be interpreted as ideology, particularly in the characteristic feature of denying its material basis. 

"We are indebted to Pascal...for the wonderful formula which will enable us to invert the order of the notional schema of ideology. Pascal says, more or less: 'Kneel down, move your lips in prayer, and you will believe.' " (Louis Althusser, "Ideology and Ideological State Apparatuses" in Zizek, Mapping Ideology, p. 127) Slavoj Zizek glosses this passage to say "kneel down and you shall believe you knelt down because of your belief. ...that the 'external' ritual performatively generates its own ideological foundation." (Intro, pp 12-13) 

Another version of this apparent reversal of cause and effect can be seen in descriptions of the physical expression of emotional states. Not only is expression (such as breathing rate, tension of the facial muscles etc.) the physical manifestation of a mental process, but the emotion can be stimulated by the physical expression itself.