What is Gatsby's "incorruptible dream"?

The phrase is from an observation that Nick makes near the end, when people are leaving yet another extravagant Gatsby party:

The lawn and drive had been crowded with the faces of those who guessed at his corruption—and he had stood on those steps, concealing his incorruptible dream, as he waved them good-by.

Nobody knows what Gatsby does for a living. Most of it is probably illegal (bootlegging, for starters). The partygoers might guess at Gatsby's occupation, but they don't really care, as long as he keeps throwing the parties.
Gatsby is a true romantic. His dream must be incorruptible, because it can't actually be tainted by reality. Jay is obsessed by the idea of his true, undying love for Daisy. He deludes himself into being sure that his money will impress her. He wants to believe that he is looking toward the future, that Daisy loves him and always will. Jay cannot see that his whole scheme is about the past, his doubt that she ever loved him, and his personal and class rivalry with her elite world (and with Tom in particular).
Nick assesses much of this accurately: Jay

wanted nothing less of Daisy than that she should go to Tom and say: ‘I never loved you.’

Nick has an inkling of Daisy's shallowness, but he never suspects that it will be a factor in Gatsby's downfall, or that the "incorruptible dream" will stay with Jay until his premature, tragic death.