Risk is defined as unknowns that have measurable probabilities, while uncertainty involves unknowns with no measurable probability of outcome. These concepts are related, but not the same.
Uncertainty and risk are closely related concepts in economics and the stock market. The definitions of risk and uncertainty were established by Frank H. Knight in his 1921 book, "Risk, Uncertainty, and Profit," where he defines risk as a measurable probability involving future events, and he argues that risk will not generate profit. Risk is calculated using theoretical models, or by calculating the observed frequency of events to deduce probabilities.
Uncertainty is not quantifiable because future events are too unpredictable, and information is insufficient. The uncertainty of the event is not something that can be calculated using past models. Though randomness of events underlies both principles, it is important to distinguish the differences as they relate to investments. An investor has the opportunity to calculate the risks by deducing past probabilities to protect his or her investment portfolio. Uncertainty is not quantifiable and therefore does not offer the same opportunity to protect an investment. Both principles work in tandem and do apply when in investing situations, or even prospects of investing on the stock market.