A network partition refers to the failure of a network device that causes a network to be split.
For example, in a network with multiple subnets where nodes A and B are located in one subnet and nodes C and D are in another, a partition occurs if the switch between the two subnets fails. In that case nodes A and B can no longer communicate with nodes C and D.
Some systems are partition-tolerant. This means that even after they are partitioned into multiple sub-systems, they work the same as before.
As a CAP trade-off
The CAP Theorem is based on three trade-offs: Consistency, Availability, and Partition tolerance. Partition tolerance, in this context, means the ability of a data processing system to continue processing data even if a network partition causes communication errors between subsystems.