An event is something that happens in the business. An event affects the flow of a business process, either kicking it off, or impacting it in some way. An event can impact a business process in three different ways:

An event can start the process flow. According to BPMN notation, a start event is denoted by a circle.

An event that happens during the course of a process flow is called an intermediate event. According to BPMN notation, an intermediate event is denoted by a circle with a double band.

An event can be the end result of a process flow. According to BPMN notation, an end event is denoted by a circle with a dark bold outline.

You can also specify more complex events, such as times, business rules, and errors.