Network
East-west traffic
Network traffic between systems inside a data centre or cloud environment, as opposed to north-south traffic entering or leaving it.
North-south traffic crosses the edge (users to your load balancer, your services to the internet). East-west traffic stays inside: service to service, service to database, host to host. In microservice architectures it is by far the larger share.
It matters because perimeter controls never see it, so it was historically unencrypted and unfiltered, which is exactly the path lateral movement takes. Controls are mTLS (often via a service mesh), microsegmentation and internal flow monitoring.