Broadcast domain
The set of devices that all receive each other's broadcast traffic. A switch (without VLANs) is a single broadcast domain; a VLAN creates a separate domain; a router stops broadcasts and thereby forms the boundary between domains.
Mechanic: when a device sends a broadcast (e.g. an ARP query or a DHCP discover), it reaches everyone in the same broadcast domain. Switches forward broadcasts to all ports (within the same VLAN); routers don't → the router (or VLAN boundary) bounds the domain. Why it matters: large broadcast domains are bad — broadcasts load every device's CPU, and the more devices the more "noise", with the risk of broadcast storms (worsened by loops). That's why you segment large networks into several VLANs/subnets so each broadcast domain stays reasonably small. Distinct from a collision domain (an older, hub-related concept). A fundamental design consideration in network segmentation. Related to collision domain and storm control.