Probability. How likely something is to happen. Many events can't be predicted with total certainty. The best we can say is how likely they are to happen, using Probability: Types of Events · Probability Line · Activity: An Experiment with a. Probability GCSE Maths revision, covering probability single & multiple events, the rules of probability and probability trees, including examples and videos. A probability is a way of assigning every event a value This mathematical definition of probability can extend to.