DNS and domains
Nameserver
A server that answers DNS queries for a zone with authority. A domain needs at least two, and they must all serve the same data.
The nameservers for a domain are the servers that hold its records and answer for them authoritatively. The registry publishes their names in the parent zone (the NS records for example.lu inside .lu); the zone itself repeats them. The two lists should match.
Most registries require at least two, on different networks, so one failure does not take the domain down. All of them must serve the same zone. A server that answers from an older copy makes some visitors get stale records at random, since resolvers pick any of the listed servers. The zone's serial number, in the SOA record, is how you tell: every server should report the same one.
A nameserver that is listed but does not answer, or answers without authority, is a "lame delegation". It slows every lookup and, if the other servers are also unreachable, makes the domain disappear. Changing nameservers is done at the registrar; the old ones should keep serving the zone until the change has propagated.