Ideology is a system of ideas and ideals — especially an organized body of doctrine held by a group, class, or culture that both explains how the world works and prescribes how it ought to be arranged.
What separates an ideology from a philosophy or a bundle of opinions is systematicity joined to an action orientation. An ideology supplies a diagnosis and a program together, which is why an adherent's position on a question they have never considered is often predictable from the doctrine alone. A second, older sense survives in academic usage — the study of the origin and nature of ideas — and a third, frankly pejorative sense means visionary theorizing detached from practice.
The word is one of the rare ones with a datable birth. The French idéologie was coined in 1796 by Antoine Destutt de Tracy for a proposed empirical science of ideas, tracing concepts back to sensation. The project did not survive its politics: Napoleon used idéologue as an insult for theorists whose schemes ignored how the world actually ran, and the pejorative shadow has never fully lifted. Marx and Engels then gave the term its most consequential turn, treating ideology as belief shaped by material interest — the ruling ideas of an age as the ideas of its ruling class — after which "ideology" named distortion at least as often as doctrine.
Both inheritances are live in ordinary English. "Political ideology" is neutral and descriptive; calling a position "ideological" accuses it of running on doctrine rather than evidence. 📝Neoliberalism illustrates the split — a policy program to its proponents, an ideology to its critics, with the choice of label doing much of the argument.