Dictionary.net

/bak'w*d k*m-bat'*-bil'*-tee/ (Play on "backwardcompatibility") A property of hardware or software revisionsin which previous protocols, formats, layouts, etc. areirrevocably discarded in favour of "new and improved"protocols, formats and layouts, leaving the previous ones notmerely deprecated but actively defeated. (Too often, the oldand new versions cannot definitively be distinguished, suchthat lingering instances of the previous ones yield crashes orother infelicitous effects, as opposed to a simple "versionmismatch" message.) A backward compatible change, on theother hand, allows old versions to coexist without crashes orerror messages, but too many major changes incorporatingelaborate backward compatibility processing can lead toextreme software bloat.See also flag day.