This document defines key concepts in probability, including: - Probability is how likely something is to happen, with all probabilities between 0 and 1. - Events have probabilities calculated by the number of outcomes of the event divided by the total number of outcomes. - Conditional probability is the probability of one event given that another event occurred, calculated as the probability of both events occurring divided by the probability of the first event. - Independent events do not affect each other's probabilities, while dependent events do influence each other's probabilities.