Replication lag
Also called: Eventual consistency
How far a database copy (a follower or replica) is behind the leader that accepts writes.
Definition
With asynchronous replication, followers apply the leader's changes a little later, usually milliseconds, sometimes seconds under load. Reads from a lagging copy can show a user their old data right after they saved, make new items disappear on refresh, or show replies before questions. Read-your-writes, sticky replicas and per-conversation ordering fix the anomalies users notice.
In practice
A user renames their profile, the next screen reads from a replica 2 seconds behind and shows the old name.