Probability is a measure of the likelihood of some event or outcome occuring . It is measured from zero (the event is impossible or will never occur) to 1 (it will definitely happen). For example, the porobablity of obtaining a number bigger than 4 on a single roll of a die is 1/3 (=1/6 for the 5 face plus 1/6 for the 6 face). Phenomenon, such as the roll of a die, that can be repeated the probability can be seen as representing the long term frequency of the outcome happening, but for singular outcomes, such as a potential civilisation-ending astroid collision, the philopsophical meaning is more problematic.
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