*Probability* is a branch of mathematics that assigns a number measuring the “chance” that some “event”—or any collection of outcomes of an experiment—will occur. It is a quantitative description of the likely occurrence of the event, conventionally expressed on a scale from 0 to 1. For example, a very common occurrence has a probability of close to 1; an event that is more rare will have a probability close to 0. In more common usage, the word “probability” means the chance that a particular event (or set of events) will occur—all on a linear scale expressed as a percentage between 0 and 100 percent (%). An even more detailed way of looking at probability includes the possible outcomes of a given event along with the outcomes’ relative likelihoods and distributions.