From Canterbury Uni's Computer Science Field Guide (CSFG):

Are there problems that are too hard even for computers? It turns out that there are. In the chapter on Artificial Intelligence we'll see that just having a conversation – chatting – is something computers can't do well, not because they can't speak but rather because they can't understand or think of sensible things to say. However, that’s not the kind of hard problem we’re talking about here – it's not that computers couldn’t have conversations, more that we don't know just how we do it ourselves and so we can't tell the computer what to do.

In this chapter we're going to look at problems where it's easy to tell the computer what to do – by writing a program – but the computer can’t do what we want because it takes far too long: millions of centuries, perhaps. Not much good buying a faster computer either: if it were a hundred times faster it would still take millions of years; even one a million times faster would take hundreds of years. That's what you call a hard problem – one where it takes far longer than the lifetime of the fastest computer imaginable to come up with a solution!

NCEA guide for the old standard AS91371 (2.44)