A broadcast domain is the portion of a network sharing the same layer 2 segment. In a network with a single switch without VLANs, the broadcast domain is that entire switch. In a network with multiple interconnected switches without the use of VLANs, the broadcast domain includes all of those switches.
A single broadcast domain can contain more than one IPv4 or IPv6 subnet, however, that is generally not considered good network design. IP subnets should be segregated into separate broadcast domains via the use of separate switches or VLANs. The exception to this is running both IPv4 and IPv6 networks within a single broadcast domain. This is called dual stack and it is a common and useful technique using both IPv4 and IPv6 connectivity for hosts.
Broadcast domains can be combined by bridging two network interfaces together but care must be taken to avoid switch loops in this scenario. There are also some proxies for certain protocols which do not combine broadcast domains but yield the same net effect, such as a DHCP relay which relays DHCP requests into a broadcast domain on another interface. More information on broadcast domains and how to combine them can be found in Bridging.