East-west movement is lateral network traffic between internal systems, such as a workstation reaching a file server or one controller talking to another segment. It is different from north-south traffic, which enters or leaves the network at the perimeter. In security terms, east-west movement matters because once an attacker gets inside, internal connections can let them spread, discover assets, steal data, or reach high-value systems.
Defenders try to limit this movement with segmentation, access controls, firewalls, and strict zone-and-conduit rules. In OT and healthcare networks, it is especially important because shared segments can let compromise jump between enterprise IT, medical devices, and industrial systems. Good monitoring focuses on actual flows, not just diagrams: if east-west traffic is wider than expected, it may indicate misconfigured segmentation or active lateral movement by an intruder.