Technical
Nameserver
Also called NS.
Nameserver is a server that holds a domain's DNS records and answers queries about them. The set your registrar lists for the domain decides where those answers come from.
How it is measured
`dig NS example.com +short` lists them, such as `ns1.provider.net`. Compare what the registrar shows with what the TLD servers return (`dig NS example.com @a.gtld-servers.net`). Any difference is a delegation problem. Aim for at least two, on separate networks.
Query each nameserver directly for the same record, for example `dig @ns1.provider.net A example.com`, and compare. Different answers mean the zones are out of sync.
Worked example
A shop moves DNS from its registrar to a managed DNS provider. It copies the zone, then changes the nameservers at the registrar. The NS records carry a TTL of 172,800 seconds at the TLD, so some resolvers keep using the old nameservers for up to two days.
The old zone was left running with an outdated A record, so visitors on those resolvers reach the old server on Wednesday. The owner keeps the old zone updated for two days and only then deletes it.
How it differs
A nameserver holds the records. DNS is the system that lets resolvers find and ask it. DNS also covers caching resolvers, and the nameserver is the authoritative end of that chain.
Common errors
Changing nameservers before copying every record, especially MX. Leaving the old and new sets mixed at the registrar. Running only one nameserver. Putting both on the same network. Deleting the old zone at once. Forgetting glue records when a nameserver lives inside the domain it serves.
In practice
Export the full zone before any change, lower the TTLs, move the records, and switch NS. Keep the old zone alive for 48 hours. After the switch, query every new nameserver and compare the answers.