red flag

Questions about red flag

What does red flag mean?

red flag means "a flag that serves as a warning signal". That is the primary sense; the word has 3 in total, listed in full on this page.

Is red flag a common word?

red flag is banded "Familiar" — widely understood, used when the subject calls for it. The band comes from how often the word appears in a large corpus of everyday English, not from an editor's impression.