Simple definition
A broadcast domain is the group of Layer 2 devices that receive an Ethernet broadcast frame; each VLAN normally forms a separate domain.
Purpose and use
A broadcast domain identifies the Layer 2 interfaces that may receive an Ethernet broadcast frame.
How does it work?
A switch floods these frames within the relevant VLAN according to active ports and rules. A router normally does not forward that broadcast into another network.
Example and verification
For failed local discovery, compare both devices' VLANs and the path through access links and trunks.
Points to watch
Sharing an IP subnet does not guarantee Layer 2 continuity. A loop or excessive broadcasts can disrupt the whole domain.
In short
The broadcast domain describes Ethernet scope; routing and access controls govern communication between domains.