When repeated experiments are carried out under certain conditions, some events will inevitably occur in each experiment. This kind of event is called an inevitable event, or simply an inevitable event.

The probability of a necessary event is 1, but an event with a probability of 1 is not necessarily a necessary event. X is a continuous random variable, and the probability of sampling any finite point in this space is 0. If the finite points are excluded from the whole sample space, the probability of getting' non-finite points' is still 1. (It can be analogized that there are a finite number of discontinuous points in the high-number integral that do not affect the integral value.