What does 'eventual consistency' concretely mean in a system built with an Axon aggregate and a projection, and how should an API account for it?

Concretely, it refers to the small but real gap in time between an aggregate applying an event and the projection's @EventHandler method reacting to that event and updating its read model table. For most systems that gap is on the order of milliseconds, but it is never truly zero, since the event has to be published, delivered, and processed by the projection before the read model reflects it. If client code immediately issues a read right after a write completes, there's a genuine chance it reads stale or missing data, because the projection hasn't caught up yet. APIs built on this pattern should be designed with that window in mind — for example, having the write endpoint return enough information for a client to know a write was accepted without assuming an immediately following read will already reflect it, rather than treating a write as instantly visible everywhere.