Simple definition
A replication partner is a domain controller that exchanges directory changes with another controller.
Purpose and use
An AD replication partner is a controller with which another exchanges changes for the relevant directory partitions.
How does it work?
Connections have a replication direction, and shared partitions determine which data can synchronize.
Example and verification
For delays, inspect inbound partners, last successful exchanges and errors for each partition separately.
Points to watch
Controllers in the same domain do not necessarily exchange every change directly; topology can use intermediary controllers.
In short
Evaluate a partner together with exchange direction, partition and propagation path.