Broadcast domains - Definition from the Hotspot Shield Glossary
A broadcast domain is a single part of a computer network that has set boundaries defined by network devices. This part of a network or division is logical and all devices/nodes in the network can be reached through broadcasting at the datalink layer.
Broadcast domains can be established via a simple Ethernet connection and provide reliable communication between nodes. Devices included in the Broadcast Domain are usually connected with network devices like routers and switches. For example, several computers connected to the same router would be classed as being part of a broadcast domain. The intermediary devices such as switches and routers control the flow of data in a broadcast domain.