backward combatability

backward combatability /bak'w*rd k*m-bat'*-bil'*-tee/ n. 
   [CMU, Tektronix: from `backward compatibility'] A property of
   hardware or software revisions in which previous protocols,
   formats, layouts, etc. are irrevocably discarded in favor 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 backwards compatible change, on the other
   hand, allows old versions to coexist without crashes or error
   messages, but too many major changes incorporating elaborate
   backwards compatibility processing can lead to extreme {software
   bloat}.  See also {flag day}.