NXDOMAIN, Non-Existant Domain, is a DNS server error response which means that a domain either do not exist or look like they due not exist due to technical issues (misconfigured name servers etc).
As an example,
nslookup doesnotexist.linuxreviews.org will report that
** server can't find doesnotexist.linuxreviews.org: NXDOMAIN
Florian Obser shared this slide in the DNS devroom at Fosdem 2020:
It turns out that a vast majority of domain name queries on the Internet are for non-valid domain names.
Some bigger ISPs, notably American ones, practice DNS hijacking of DNS queries who should return NSDOMAIN responses. The fake responses given will typically point to landing pages with advertisements on them.