What Does Warm Server Mean?
A warm server is a backup that is booted periodically for updates and is often is used for replication and mirroring. A computer server backup exists in case the main server crashes or is destroyed, such as in a natural disaster. A warm server is rebooted on a regular basis to sync updates with the server that is backed up.
Techopedia Explains Warm Server
During disaster recovery, warm servers may have been configured but need upgrades to be used. Warm server implementation costs fall between the time and resources associated with cold and hot servers.
Microsoft provides free software licenses for cold servers intended for disaster recovery, but warm or hot server licenses must be purchased.