Definition - What does Primary Server mean?
A primary server is a server that acts as the first source for Domain Name System (DNS) data and responds to queries. It can be contrasted to the secondary server, which acts like the primary server but does not have the same access to data.
Techopedia explains Primary Server
Experts point out that one of the main differences between a primary server and a secondary server is that while the primary server gets data from local files, the secondary server must ask the primary server for data. This can be explained with a simple metaphor showing how a secondary server might support a primary server. If the primary server is temporarily busy, the secondary server can act as a backup resource. For this reason, experts recommend that IT systems have two servers, a primary and a secondary. One issue with this kind of setup is provisioning both of the servers correctly. One way to do this is to use a server farm that will distribute workloads to servers accordingly.