<humour> /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.