<jargon> A software change that is neither forward- nor
backward-compatible, and which is costly to make and costly to
reverse. E.g. "Can we install that without causing a flag day
for all users?"
This term has nothing to do with the use of the word flag to
mean a variable that has two values. It came into use when a
massive change was made to the Multics time-sharing system
to convert from the old ASCII code to the new one; this was
scheduled for Flag Day (a US holiday), June 14, 1966.
See also backward combatability, lock-in.
[Jargon File]
(1998-01-15)
Nearby terms:
FL « F+L « flag « flag day » FLAIR » flaky » flamage
