The event is the set of data recorded by an experiment at a given time. It can be triggered by a set of time coincidence of fast detectors signal or by the accelerator/collider timing system.
It is also a sample of a given process. for example: an event of the process e+ e- -> e+ e- shows, in the center of mass, the final electron and positron back to back. The angle with the initial collision direction is random and will be different from one event to the other.