red flag

Short definition from Wiktionary

Noun

1. 1
   A flag that is red, especially associated with leftist revolutionary ideologies.
2. 2
   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).
3. 3
   A cue, warning, or alert; a sign or signal that something is wrong.
4. 4
   Something that will enrage a particular person; a red rag.
5. 5
   A flag formerly used by a railway guard to warn the driver not to start the train.

Verb

1. 1
   To mark or identify something as suspicious, problematic or worthy of further scrutiny or immediate action.
2. 2
   To halt a race or session immediately due to unsafe conditions.

Proper noun

1. 1
   A series of advanced, realistic air combat training exercises conducted by the U.S. Air Force.
2. 2
   A socialist song, emphasising the sacrifices and solidarity of the international labor movement.
3. 3
   Synonym of red flag warning