What is a group of nodes that hear each other traffic?

A group of nodes that hear each other's traffic is typically referred to as a "broadcast domain." In a broadcast domain, all devices within that domain can communicate directly with each other without needing to route through a different network segment. This is common in local area networks (LANs) where devices can send broadcast messages that all other devices in the domain can receive. The size of a broadcast domain can be affected by network devices like routers and switches, which can segment or expand these domains.