The Jargon File (version 4.4.7, 29 Dec 2003):

rot13
 /rot ther'teen/, n.,v.

    [Usenet: from ?rotate alphabet 13 places?] The simple Caesar-cypher
    encryption that replaces each English letter with the one 13 places forward
    or back along the alphabet, so that ?The butler did it!? becomes ?Gur
    ohgyre qvq vg!? Most Usenet news reading and posting programs include a
    rot13 feature. It is used to enclose the text in a sealed wrapper that the
    reader must choose to open ? e.g., for posting things that might offend
    some readers, or spoilers. A major advantage of rot13 over rot(N) for
    other N is that it is self-inverse, so the same code can be used for
    encoding and decoding. See also spoiler space, which has partly displaced
    rot13 since non-Unix-based newsreaders became common.


The Free On-line Dictionary of Computing (30 December 2018):

rot13

   /rot ther'teen/ [Usenet: from "rotate alphabet 13 places"],
   v. The simple Caesar-cypher encryption that replaces each
   English letter with the one 13 places forward or back along
   the alphabet, so that "The butler did it!" becomes "Gur ohgyre
   qvq vg!"  Most Usenet news reading and posting programs
   include a rot13 feature.  It is used to enclose the text in a
   sealed wrapper that the reader must choose to open - e.g. for
   posting things that might offend some readers, or spoilers.
   A major advantage of rot13 over rot(N) for other N is that it
   is self-inverse, so the same code can be used for encoding and
   decoding.

   [Jargon File]