Crossword Dictionary
red flag
red flag - n
a flag that serves as a warning signal; "we didn't swim at the beach because the red flag was up"
the emblem of socialist revolution
A red flag could either be a literal red flag used for signaling or, as a metaphor, a sign of some particular problem requiring attention.
The term and the expression "to raise the red flag" come from various usages of real flagsthroughout history. A red flag is frequently flown by armed forces to warn the public of live fire exercises in progress, and is sometimes flown by ships carrying munitions (in this context it is actually the flag for the letter B in the international maritime signal flag alphabet, a red swallow-tailed flag). In many countries a red flag is flown to signify that an outdoor shooting range is in use.
The United States Air Force refers to its largest annual exercise as Red Flag operation. Red flags are used for various signals in team sailing races (see Racing Rules of Sailing). A red flag warning is a signal of high wildfire danger, and a red flag on the beach warns of dangerous water conditions (double red flags indicate beach closure). Red flags of various designs indicate dangerous wind and wave conditions for mariners.