Acronym for a DNS Resource Record.
Some DNS Resource Record Types:
A more comprehensive list of RRs is at: http://www.dns.net/dnsrd/rr.html
InNeedOfRefactor: fold all the pages for individual RR types into this one; none of them is long, and using "an A RR" gives better context for links than "an A? record".
No page links to RR.