Definition of "red flag" in English

Noun

1. A flag that is red, especially associated with leftist revolutionary ideologies. A flag used as a signal that the race must be stopped immediately (and possibly restarted).

2. A flag used as a signal that the race must be stopped immediately (and possibly restarted).

3. A cue, warning, or alert; a sign or signal that something is wrong.

   • Improper formatting is a red flag to look for other problems with the data.

4. Something that will enrage a particular person; a red rag.

5. A flag formerly used by a railway guard to warn the driver not to start the train.