What refers to the probability that something will NOT happen?

The phrase that refers to the probability that something will not happen is known as the complement of an event. In probability theory, if an event has a probability of occurring, the complement reflects the likelihood of that event not occurring. For example, if the probability of rain tomorrow is 70%, then the complement, or the probability that it will not rain, is 30%. This concept is fundamental in probability, as it allows for a complete understanding of likelihood by considering both the occurrence and non-occurrence of events.

The use of the term "complement" is standard in mathematics and statistics and is pivotal when calculating probabilities, especially in problems where you want to determine the likelihood of at least one event occurring or managing multiple independent events. Understanding complements enhances decision-making processes in both theoretical and practical applications of probability.