PTR record
Reverse DNS. Address back to a name, under in-addr.arpa or ip6.arpa.
PTR maps an IP, encoded as a name in the reverse tree, to a hostname. Mail receivers still look at it. Humans use it when an alert is only an address.
Forward and reverse should agree if you care about mail or simple trust heuristics. Anyone can name reverse space they own. A PTR is not proof they own the forward domain.
See also: A record, AAAA record, MX record.