An event indicates a change in an environment such as an AWS environment, a SaaS partner service or application, or one of your applications or services. The following are examples of events:

Amazon EC2 generates an event when the state of an instance changes from pending to running.

Amazon EC2 Auto Scaling generates events when it launches or terminates instances.

AWS CloudTrail publishes events when you make API calls.

You can also set up scheduled events that are generated on a periodic basis.

For a list of services that generate events, including sample events from each service, see Events from AWS services and follow the links in the table.

Events are represented as JSON objects and they all have a similar structure, and the same top-level fields.

https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-events.html