Definition
A nameserver is a server that stores DNS records for a domain and answers queries about them. Every domain has at least two nameservers (for redundancy) listed in its NS records. Your domain registrar either provides nameservers or lets you specify custom ones. When you change DNS providers (e.g., to Cloudflare), you update your domain's nameservers at the registrar.
Use the tool: NS Lookup — check nameserver for any domain.