Probability measures the likeliness that a particular event will occur. When mathematicians (us, for now!) speak of an event, we are referring to a set of potential outcomes of an experiment, or trial, to which we can assign a probability of occurrence.

Probabilities are expressed as a number between 0 and 1 (or as a percentage out of 100). An event with a probability of 0 denotes an impossible outcome, and a probability of 1 describes an event that is certain to occur.

The canonical example of probability at work is a coin flip. In the coin flip event, there are two outcomes: the coin lands on heads or the coin lands on tails. Pretending that coins never land on their edge (they almost never do), those two outcomes are ...