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.
I have found this most wonderful (if fairly lengthy) article, and thought you would enjoy having it brought to your attention.
It is so good, I think we should include it among the references in the "Politics is the Mind Killer" wiki page. But, before that, I submit it to you, and ask you: is there anything in this article that would warrant its exclusion from this site? I mean, besides the fact that it is about politics, and written by a notorious Social-democrat (And is that in itself grounds for exclusion?).