Full Idea
Adorno defines ideology as 'socially necessary illusion' or 'socially necessary false-consciousness' (and the young Habermas accepted something like this definition).
Gist of Idea
Ideology is 'socially necessary illusion' or 'socially necessary false-consciousness'
Source
report of Theodor W. Adorno (works [1955]) by James Gordon Finlayson - Habermas Ch.1:11
Book Reference
Finlayson,James G.: 'Habermas' [OUP 2005], p.11
A Reaction
The marxism seems to reside in the view that such things are always 'false'. If they gradually became 'true', would they cease to be ideology? Is it impossible for widespread beliefs to be 'true'?