Ideology is the mental lens through which the subject interprets his social-material condition. Ideology encompasses such fields as art, family, culture, religion, philosophy, media, and education.
Karl Marx, Friedrich Engels, Vladimir Lenin and others, like Mao Zedong, all discuss ideology within the context of the class struggle. However, a deeper dive into the function of ideology within capitalism and its mechanical role in the reproduction of a mode of production was most prominently outlined by Louis Althusser in his essay Ideology and Ideological State Apparatuses (1970).
Rather than seeing ideology merely as a set of ideas or a philosophy, Althusser defined it as a material practice that constitutes individuals as subjects. Ideology, for Althusser, is a 'lived' reality, a part of our everyday life and our perception of the world, not something that can simply be dispelled or 'seen through'. In his conceptualization, ideology works through ideological state apparatuses, the various institutions like family, education, media, etc., that reproduce ideology and, in doing so, reproduce the existing social order.