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.
Used on pages 41, 42, 45, 50, 76, 77, 140, 182, 207, 235, 236, 237, 317, 322, 323, 450, 464, 472, 473, 520, 541