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 a mere set of ideas or beliefs, Althusser defines it as a material practice of complex systems responsible for interpellating the subject into specific roles within the existing social order. Ideology is an "actively lived reality", and not something that can simply be dispelled or 'seen through'; in this conceptualization, ideology works through ideological state apparatuses, or various institutions like family, education, media, etc., that reproduce ideology and, in doing so, reproduce the existing social order.