What is risk?

For some reason I've been involved in discussions, over the last couple of days, as to what risk is. Some say its an event, some describe it as an emergent process from a complex organisation (Milliman), ISO31000:2009 say it's the effect of uncertainty on objectives. I think we need to make sure that we don't have a category error creeping into our thinking. A suitably generic notion of risk is that it's exposure to something that may be disadvantageous in some way. We can then start from here.