Broadcast Domain is the domain of a Network in which broadcasts can be sent by a node and received by all other nodes connected in that network domain. The broadcast is done at the Data Link Layer (Layer 2) of the OSI Layers. Special Layer 2 address is allocated for broadcast known as broadcast address. If a computer needs to send broadcast to other nodes connected, the broadcast layer 2 address is used as the destination address in the layer 2 frame.
A switch and hub are single broadcast domain network devices. All connected devices in switch or hub will receive broadcast when sent by a node.
A router is a multi-broadcast domain network device. Each layer 3 interface becomes a broadcast domain.