Actually Orwell has used the term "Collectivism" in other contexts. It's about as vague a term as "liberalism", "elitism" or "republicanism", and takes specific meanings in specific contexts.

Here is an article that discusses it from many different viewpoints and tries to see the big picure

Having read that, I'm not sure Orwell's "Nationalism" is isometric to the general concept of "Collectivism", since "Patriotism", which, according to him, is "Nationalism"'s opposite, fits there too. Somehow it seems more specific: it's about dividing the world in power units and rooting for one and/or booing another. You could say it's about dividing the world in sports teams.

Which is why I always despised spectacle sports with all my soul.