For the last decade or so, east-west traffic was just the traffic within a container cluster. That is the communication that happens between microservices, usually in Kubernetes. Service Mesh is an example of a solution that emerged to address the special need for security and visibility of east-west traffic.
But with organizations not only operating applications in a multicloud environment but deploying multiple components of an application in multiple environments (a.k.a. hybrid apps), east-west traffic is no longer contained by the cluster. It reaches across network boundaries, across the big bad Internet.
