red flag

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red flag (plural red flags)

  1. (idiomatic) 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.
    • 2012, Alan Rappeport, Financial Times, Eli Lilly settles bribery charges
      The US Securities and Exchange Commission alleged that Eli Lilly subsidiaries funnelled millions of dollars to government employees in other countries to secure business and did little to follow up on red flags.
  2. (rail transport) In the United Kingdom and some other countries, a flag formerly used by a railway guard to warn the driver not to start the train.
  3. (politics) A common symbol, usually in the form of a banner, for leftist revolutionary ideologies, especially socialism.