Definition
NS records identify the authoritative nameservers for a domain — the servers that hold the definitive DNS records. When you change DNS providers, you update NS records at your registrar. NS records cannot be self-referential: you need at least two NS records (primary and secondary) for redundancy. NS changes propagate from the TLD registry and take 24-48 hours.
Use the tool: NS Lookup — check ns record for any domain.