The practical difference between the two categories, risk and uncertainty, is that in the former the distribution of the outcome in a group of instances is known (either through calculation a priori or from statistics of past experience), while in the case of uncertainty this is not true, the reason being in general that it is impossible to form a group of instances, because the situation dealt with is in a high degree unique.
Risk, Uncertainty and Profit
Frank Knight 1921
Risk of falling off your bike = known potential outcomes.
Uncertainty of the future of European Union government = unknown potential outcomes.
Risks are therefore the known unknowns.
Uncertainties are the unknown unknowns.
Startups operate in a world of uncertainty and they need to identify risks to their business model, address these risks and reduce uncertainty.