Definitions
1.NounA 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.NounA flag used as a signal that the race must be stopped immediately (and possibly restarted).
3.NounA 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.NounSomething that will enrage a particular person; a red rag.
5.NounA flag formerly used by a railway guard to warn the driver not to start the train.