Network Partition is a division of a network into a separate subnets, either by design or by network failure.
When a network partition occurs, all traffic sent from a component in partition A to components in partition B will be lost. Any pattern of message loss can be modeled as a temporary network partition, lasting long enough for the message in question to be lost.
Network partitions can't be experienced only by single-node, monolythic systems. Even in case of systems like these, network partition can happen between the monolith and its client, still causing a problem.
The cause of Network Partition is not just dropping packets - even when a node crash occurs, it can be said that the network was partitioned in two pieces - failed node, and the rest of the network.