What is the difference between risk and uncertainty in economics?

This is a question of definitions, and it depends on whom you ask, but in general risk is measurable uncertainty. To avoid ambiguity, risk is redundantly called calculated risk or Knightian risk by some. Uncertainty that is immeasurable is called Knightian uncertainty, after Chicago economist Frank Knight who distinguished between risk and uncertainty in 1921 in Risk, Uncertainty, and Profit: Uncertainty must be taken in a sense radically distinct from the familiar notion of Risk, from which it has never been properly separated. … The essential fact is that 'risk' means in some cases a quantity susceptible of measurement, while at other times it is something distinctly not of this character; and there are far-reaching and crucial differences in the bearings of the phenomena depending on which of the two is really present and operating.... It will appear that a measurable uncertainty, or 'risk' proper, as we shall use the term, is so far different from an unmeasurable one that it is not in effect an uncertainty at all. http://en.wikipedia.org/wiki/Knightian_uncertainty