Reverse DNS is the process of using DNS to translate IP addresses to hostnames.
Reverse DNS is the opposite of forward DNS, which is used to translate hostnames to IP addresses.

So, to reverse-resolve a known IP address is to look up what host and domain name belongs to that IP address.
A reverse lookup is often referred to as reverse resolving, or more specifically reverse DNS lookup, and is accomplished using a "reverse IN-ADDR entry" in the form of a PTR record.

Uses
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The most common uses of the reverse DNS are:
===  * The original use of the rDNS was primarily for network troubleshooting tools, such as traceroute, ping, and the "Received:" trace header field for SMTP e-mail, web sites tracking users (especially on Internet forums), etc.

===  * One e-mail anti-spam technique is to check the domain names in the rDNS to see if they are likely from dialup users, dynamically assigned addresses, or home-based broadband customers. Since the vast majority, but by no means all, of e-mail that originates from these computers is spam, many mail servers also refuse e-mail with missing or "generic" rDNS names.

===  * A Forward Confirmed reverse DNS (FCrDNS) verification can create a form of authentication showing a valid relationship between the owner of a domainname and the owner of the server that has been given an IP address. While not very thorough, this validation is strong enough to often be used for whitelisting purposes, mainly because spammers and phishers usually can't pass verification for it when they use zombie computers to forge domains.

===  * Many SSH clients hang for a pre-determined timeout period upon connecting to SSH server machines when the client does not have a properly-configured rDNS "PTR" entry. Adding an rDNS "PTR" entry for a client that connects to an SSH Daemon is known to speed up the lag between connection and logon in certain cases.