P(AB) indicates the probability that events A and B occur at the same time, and P(AUB) indicates the probability that at least one of events A and B occurs.

The word "probability" comes from the Latin word "probability itas" and can also be interpreted as "integrity". Integrity means "integrity and honesty". In Europe, it is used to express the authority of witness testimony in court cases, which is usually related to the reputation of witnesses. In short, it is different from the meaning of probability "possibility" in the modern sense.

Probability, also known as "probability", reflects the probability of random events. Random events refer to events that may or may not occur under the same conditions. For example, it is a random event to randomly select one from a batch of goods with genuine products and defective products, and "what is extracted is genuine products".

Suppose a random phenomenon is tested and observed n times, in which event A appears m times, that is, its frequency is m/n. After a lot of repeated experiments, m/n is often closer to a constant (see Bernoulli's law of large numbers for details). This constant is the probability of the occurrence of event A, which is usually expressed by P (A).