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 Chap. 3: pages 41, 42, 45, 50; Chap. 4: pages 76, 77; Chap. 7: page 140; Chap. 9: page 182; Chap. 10: page 207; Chap. 11: pages 235, 236, 237; Chap. 14: pages 317, 322, 323; Chap. 18: page 450; Chap. 19: pages 464, 472, 473; Chap. 21: page 520; Chap. 22: page 541