Ideology in the philosophical sense is not a synonym for political opinion. It names a set of beliefs that serves an interest while appearing to its holders as simply how things are.

Marx's version

Marx used the term for the ideas that arise from and sustain a particular economic arrangement. The ruling ideas of any age, on his account, are the ideas of its ruling class — not because of conspiracy, but because the arrangements that produce a society's material life also shape what seems obvious in it.

The important feature is invisibility. An ideology that announced itself would have no force. It works by making a historical arrangement look like the natural order, so that questioning it seems eccentric rather than political. This connects to alienation and to commodity fetishism, where relations between people appear as relations between things.

False consciousness

The associated claim is that people can hold beliefs against their own interests without being deceived by anyone in particular. The phrase invites an obvious objection: who is in a position to say what someone's real interests are, and what makes that view exempt from the same analysis?

Later writers took the objection seriously. Gramsci's hegemony describes consent secured through culture rather than force, and does not require anyone to be mistaken about their interests.

In critical theory

Critical theory developed the analysis without the economic determinism. For Adorno and Horkheimer the culture industry sells the appearance of choice while reinforcing the arrangement. Marcuse argued that satisfied manufactured needs absorb opposition more effectively than repression could.

The self-application problem

Every account of ideology has to say why it is not itself one. Answers include grounding critique in the interests of those the arrangement harms, or in conditions of undistorted discussion. None is uncontested, and the difficulty is the reason the concept has to be used carefully rather than as an accusation. See belief systems and revealed preference.