Network partition in the context of distributed system. For a subnet network partition, see Network - Partition
A network partition refers to a network split between nodes due to the failure of network devices. Example: When switch between two subnets fails, there is a partition between nodes.
For a reduction in availability to be perceived, there must be both:
As networks become more redundant, partitions become an increasingly rare event and this combination of events even rarer than other causes of system unavailability.
Because partition have a real negligible impact on availability, it is very possible to have a system that guarantees consistency, high availability and partition tolerance. See Distributed Database - CAP Theorem (Consistency, Availability, Partition Tolerance)