Computing term for a binary indicator or status marker.
The program sets an error flag so that processing can branch later.
In this program, processing stops when an error flag is set.
foreshadowing; sign of things to come
Colloquial use, often in discussions of stories, games, or real-life situations, referring to an event or line that hints at a future development.
That line is a death flag, isn't it.
Him suddenly becoming kind might be a sign of something.
The protagonist who said “I’ll definitely make it back alive” ended up fulfilling that ominous line, just as expected.
He deliberately made a weak-sounding comment to try to break the ominous flag.
parent intrusion flag; parent alert
edge flag
sign flag
idle flag
debug flag
flag sequence
From English 'flag'. The computing sense is a direct borrowing; the colloquial foreshadowing sense developed in Japanese from the idea of a flag as a marker or signal.