A broadcast is a logical division of a computer network in which all nodes can reach each other by broadcast at the data link layer.
In terms of current popular technologies: any computer connected to the same Ethernet switch is a member of the same broadcast domain. Further, any computer connected to the same set of inter-connected switches is a member of the same broadcast domain. Routers and other layer 3 devices form boundaries between broadcast domains.
Layer 2 of the OSI model
- A switch creates an entire broadcast domain (provided that there's only one VLAN) since broadcasts are a layer 2 concept (mac address related)
- Routers don't forward layer 2 broadcasts, hence they separate broadcast domains