A Event is an object in time, or contained by The Timeline. Events can contain objects. There are two exact and concise interpretations of what an event is.
In A-Theory of time
According to The A-Theory, the passage of time is a very real and inexorable feature of the world, and not merely some mind-dependent phenomenon. In other words, the past, present ,and future are distinct. The flow and passage of time is real, and that temporal becoming is an objective feature of reality. As the smallest unit of time is the Planck-time, all objects, and events, for the duration of their existence, exist in each Planck-time. Therefore, an event encompasses everything with the duration of the event within each Planck-time.
An event can be an instance or a continuation of action with a finite time span. Existence of beings below the level of the Creator can thus be considered events, however the existence of the Creator cannot be considered as an event as:
1: The Creator is not instantiated.
2: The Creator existed, exists, and will exist for an infinite time span by definition.
The Universe is the cause of an event, as it is instantiated. So is the Megaverse, Gigaverse, and so on. The Universe is also an event, as it is undergoing the process of existence, and has a finite lifespan. All possible realms would be both caused by an event and be an event. This is the same case with all dimensions. Events are everything that has happened.
In B-Theory of time
According to B-theory of time, the flow of time is an illusion. The past, present and future are equally real, and that time is tenseless. This would mean that temporal becoming is not an objective feature of reality. Time, in this case, would be very much like the dimensions of space. Hence, an event would be a slice or a chunk in this loaf of time. Past events have never truly passed, present events are not the objective reality, and the future can exist beyond speculation, but as an holistic state. An event is now only a infinitesimal of the eternity that is the Timeline, unlike a real object in A-theory of time. All of existence is now an event, even the non-existence of objects, ideas, concepts are technically events as in the infinite span of the Timeline, the future and past are equally real. The existence of the Creator can thus be an event, as duration is not a key concept of an event.
The Universe would still be the cause of an event, and is an event simultaneously. But the non-existence of the Universe would also be an event, too. This applies for everything below the level of the Creator.