Make a yes-or-no decision with a given likelihood
The probablility P of something occurring is defined as the number of looked-for outcomes divided by the number of possible outcomes. For example, the probability that heads will show up on an unbiased coin flip is 1 (there is one instance of heads on the coin) divided by 2 (there are two possible outcomes, heads or tails), which means P = 0.5. With this definition, any probability can be described as a number in the range from 0 (impossible) to 1 (absolute certainty).