Source : Free On-Line Dictionary of Computing

backward compatibility
     
         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)