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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.
--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.