A term with an unusually twisted career. Coined by the French idéologue Destutt de Tracy around 1796 to name what he hoped would be a rigorous "science of ideas," it acquired its enduring meaning through Karl Marx and Engels's The German Ideology (written 1845–46, published 1932), where it names something like the opposite: not the science of ideas but the systematic misrecognition of social reality in the self-understanding of a ruling class, whose interests appear to it and to everyone else as universal truths.
Three uses now circulate. Pejorative / Marxist: ideology is false consciousness, a distortion of reality that serves dominant interests — typically by presenting historically specific arrangements (private property, patriarchal family, national sovereignty) as natural or eternal. Neutral / descriptive: ideology is any organized system of political beliefs — liberalism, conservatism, socialism, nationalism, and so on are all ideologies in this sense, regardless of their truth. Positive / constitutive: the view (associated especially with Althusser) that ideology is not an illusion to be stripped away but the imaginary relation through which we live our real conditions; we are all always already inside some ideology, and the question is which.
Antonio Gramsci's hegemony softened the Marxist picture by emphasizing the active, cultural work of producing and reproducing ruling-class worldviews through the institutions of civil society. The The Frankfurt School extended the analysis to mass culture. Contemporary "critical" traditions use ideology-critique to denaturalize all manner of received common sense. Critics (including some from within Marxism, such as Raymond Geuss) have worried that the concept often smuggles in an implicit claim to a non-ideological standpoint that the critic cannot actually deliver.