Osborn's words from his own book where the term was first used:

  Every day, everywhere in a democracy, juries are proving that a dozen minds can jointly judge and judge well. But that’s judicial thinking; how about creative thinking? Can a squad produce ideas? The answer is yes. Properly organized and run, a group can be a gold-mine of ideas. It was in 1939 when I first organized such group-thinking in our company. The early participants dubbed our efforts “Brainstorm Sessions”; and quite aptly so because, in this case, “brainstorm” means using the brain to storm a creative problem—and do so in commando fashion, with each stormer attacking the same objective.

So the good news is you can still brainstorm as Osborn himself intended. I suppose this once again goes to show that reading the manual is a good idea! I had no idea there was a manual for this stuff.