Source : Free On-Line Dictionary of Computing

backward combatability
     
         /bak'w*d k*m-bat'*-bil'*-tee/ (Play on "{backward
        compatibility}") A property of hardware or software revisions
        in which previous {protocols}, formats, layouts, etc. are
        irrevocably discarded in favour of "new and improved"
        protocols, formats and layouts, leaving the previous ones not
        merely deprecated but actively defeated.  (Too often, the old
        and new versions cannot definitively be distinguished, such
        that lingering instances of the previous ones yield crashes or
        other infelicitous effects, as opposed to a simple "version
        mismatch" message.)  A backward compatible change, on the
        other hand, allows old versions to coexist without crashes or
        error messages, but too many major changes incorporating
        elaborate backward compatibility processing can lead to
        extreme {software bloat}.
     
        See also {flag day}.
     
        [{Jargon File}]
     
        (2003-06-23)