"Risk" is used colloquially as a term for a danger that arises unpredictably, such as being struck by a car. Sometimes it is used for the likelihood of a particular danger or hazard, as when someone says, "You can reduce your risk of being hit by a car by crossing at the crosswalk."

When used in technical context, such as in the terms "risk assessment" or "risk management," the notion of risk is the probability or likelihood of some resulting harm (such as the likelihood of being killed by being struck by a car) multiplied by the magnitude of the harm. One can then compare, say, the average citizen’s risk of death from crossing the street with such a person’s risk of death from cancer in a given period. One could also compare the risk of harms of different magnitudes. For example, two monetary risks: the rather likely event of losing a quarter in a malfunctioning vending machine, and the comparatively unlikely loss of one’s wallet due to robbery at gun point. It may turn out that there is a greater risk of monetary loss from malfunctioning vending machines than from robbery at gun point. Notice that use of the technical sense of risk requires that one be able to meaningfully quantify the resulting harms. For many harms this is difficult to do except in an arbitrary way.