A service that routes events from Google Cloud (such as an object being created in Cloud Storage or an operation recorded in audit logs) and other sources to destinations such as Cloud Run or GKE.
On the ACE it is used so a Cloud Run service reacts automatically to an event, for example every time a file is uploaded or a VM is created, without polling.
Eventarc is a concept that can show up on the exam. Check the full guide to see it in context.