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

backward compatibility
backward compatible
backwards compatibility
backwards compatible

    Able to share data or commands with older versions of
   itself, or sometimes other older systems, particularly systems
   it intends to supplant.  Sometimes backward compatibility is
   limited to being able to read old data but does not extend to
   being able to write data in a format that can be read by old
   versions.

   For example, WordPerfect 6.0 can read WordPerfect 5.1 files,
   so it is backward compatible.  It can be said that Perl is
   backward compatible with awk, because Perl was (among other
   things) intended to replace awk, and can, with a converter,
   run awk programs.

   See also: backward combatability.

   Compare: forward compatible.

   (2003-06-23)