A nameserver is a server that responds to DNS queries with information about a domain, such as the records that direct a name to an IP address or another service. Domains can use different nameservers for different portions of their namespace, including delegated subdomains.
Nameservers matter in cyber security because control over DNS can redirect users or enable content to appear under a trusted domain without changing its main website. Attackers who alter DNS settings or take advantage of a poorly monitored delegated subzone may point subdomains to infrastructure they control. Defenders should review nameserver changes, delegation settings, and unexpected DNS records as part of routine monitoring.