Joker.com FAQ - What are 'PTR' records?

DNS PTR Records

A DNS pointer record (PTR for short) provides the domain name associated with an IP address.

A DNS PTR record is exactly the opposite of the 'A' record, which provides the IP address associated with a domain name.

DNS PTR records are used in reverse DNS lookups. When a user attempts to reach a domain name in their browser, a DNS lookup occurs, matching the domain name to the IP address.

A reverse DNS lookup is the opposite of this - it searches for a domain name with the given IP address.

This also means that PTR records can not be defined using the name servers of the domain, but have to be requested at the provider of the IP address, in case the provider supports this.