Definition - What does Secondary Server mean?
A secondary server is a type of server that serves as an addition to the primary server and is used for a variety of services. It has the same features and capabilities as the primary server and acts as a second or substitutive point of contact in case the primary server is unavailable, busy or overloaded.
A secondary server may also be called a slave server.
Techopedia explains Secondary Server
A secondary server is primarily used to provide an alternative source for the services provided by the primary server. A secondary server can be used for different services such as as a DNS server, application server, storage server and more.
For example, within DNS server infrastructure, a secondary server provides the same services as the primary server but serves different zone requests. In this way, DNS queries can be resolved much more quickly than if served by the primary server. A secondary server is often confused with a backup server, but in reality, the latter is actually a type of implementation of a secondary server.