/bak'w*rd k*m-bat'*-bil'*-tee/
n. A property of hardware or software revisions in which previous protocols, formats, and layouts are discarded in favor of "new and improved" protocols, formats and layouts. Occurs usually when making the transition between major releases. When the change is so drastic that the old formats are not retained in the new version, it is said to be "backward combatable".
See flag day.