A network partition happens when parts of a distributed system can no longer communicate with each other.

The machines may still be running. The processes may still be healthy. From the outside, both sides may even look like they are serving traffic normally.

The problem is that the system has lost the ability to coordinate across the partition boundary.

That is what makes partitions dangerous. A crashed node stops making changes. A partitioned node keeps making changes, while other nodes make different changes at the same time, and neither side knows about the other until the network heals.