"Network domain" is a term used to differentiate between multiple private computer networks within the same infrastructure. The term is unofficial and has mostly spread through word of mouth.
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Example
Half of the staff of Building A uses Network 1, 192.168.0.0/24. This network has the Vlan identifier of Vlan 10
The other half of the staff of Building A uses Network 2, 192.168.30.0/24. This network has the Vlan identifier of Vlan 20
All of the staff of Building B uses Network 3, 192.168.0.0/24. This has the Vlan identifier of Vlan 11.
The router R1 serves as the gateway for all three networks, and the whole infrastructure is connected physically via ethernet cable. Networks 2 and 3 are routed through R1 and each has full access to the other.
Network 1 is completely separate from the other two, and will not have access to either of them. Network 2 and 3 are therefore in the same network domain, while Network 1 is in its own network domain, albeit alone.
These network domains can then be suitably named to match the infrastructure terminology.
Usage
While not a recognized standard term within networking circles, the phrase "network domain" meets the need for network administrators to separate different networks which use the same IP address-range, and has thus formed the term which is now widely used in certain geographic areas.