Definition of red flag

Short definition: The word red flag means a clear warning sign that something may be wrong or unsafe. People use it to describe behaviors, situations, or signals that suggest a problem and should not be ignored.

Looking for a clear and natural explanation of the phrase red flag? The phrase red flag is commonly used in everyday conversation to describe a warning sign that something may be wrong. People use it when talking about relationships, work situations, behavior, or decisions that feel risky or unsafe. Just like a real red flag signals danger, this expression warns people to be careful and pay attention. Below, each meaning is explained in a natural and human way, so it feels easy to understand and useful in real life.

Forms: red flag, red flags
Related: warning sign n., danger signal n.
Syllable: red flag

1. A warning sign that something may be wrong.

   A red flag is a clear sign that tells you to be careful. It usually appears when a situation, a person, or a choice shows behavior that could lead to trouble later. People often talk about red flags when they notice early signs of problems.

   His constant lying was a big red flag.
   Ignoring safety rules is a serious red flag.
   The company’s lack of honesty raised a red flag.

   Synonyms: warning sign, danger signal, alert sign, caution sign, trouble sign, risk signal, problem indicator, early warning, threat sign, caution mark, risk alert, warning clue
   Antonyms: green light, safety sign, approval signal, positive sign, good sign, trust signal, clear path, safe indicator, encouraging sign, reassurance, all-clear signal, positive signal

2. A sign of unhealthy behavior in relationships.

   In relationships, a red flag refers to behavior that suggests emotional, mental, or physical harm. Things like controlling actions, constant jealousy, or lack of respect are often called red flags because they warn people to protect themselves.

   His controlling behavior was a major red flag.
   Lying early in a relationship is a red flag.
   She learned to notice red flags before getting too attached.

   Synonyms: relationship warning, toxic sign, danger behavior, unhealthy signal, abuse warning, trust issue sign, problem behavior, control signal, emotional risk sign, boundary warning, caution behavior, harm indicator
   Antonyms: healthy sign, trust signal, respect sign, supportive behavior, safe relationship sign, caring signal, positive trait, good character sign, emotional safety, mutual respect, healthy behavior, strong foundation sign

3. A sign that a decision may lead to trouble.

   A red flag can also describe something that warns you about a bad choice. It may not stop you immediately, but it suggests that you should think again before moving forward.

   The unclear contract terms were a red flag.
   Rushing into the deal felt like a red flag.
   That strange offer raised a red flag.

   Synonyms: decision warning, risk sign, caution signal, danger hint, trouble clue, warning marker, risky sign, alert indicator, problem sign, unsafe signal, threat hint, caution clue
   Antonyms: safe choice sign, green light, approval signal, clear decision, positive outcome sign, secure option, confident choice, safe path, encouraging signal, trustworthy sign, reliable indicator, good direction sign

4. A signal used to stop or warn in real situations.

   In a literal sense, a red flag is an actual flag used to warn people about danger, such as on beaches, race tracks, or construction sites. It tells people to stop, slow down, or stay away for safety.

   The lifeguard raised the red flag because the waves were too strong.
   A red flag stopped the race after the accident.
   Workers placed a red flag near the broken road.

   Synonyms: danger flag, warning flag, stop signal, safety flag, hazard marker, caution flag, alert flag, risk signal, emergency sign, danger marker, safety alert, hazard sign
   Antonyms: green flag, go signal, safety clearance, all-clear sign, permission signal, safe-zone marker, approval sign, continue signal, clear-path sign, safe indicator, positive signal, go-ahead sign

5. A phrase used to express strong concern.

   In casual speech, people use red flag to express that something feels seriously wrong. It becomes a simple way to say, “This doesn’t feel right, be careful.” The phrase helps people communicate concern quickly.

   That comment was a total red flag for me.
   The sudden change felt like a red flag.
   Everyone agreed it was a red flag.

   Synonyms: strong warning, serious concern sign, danger expression, caution phrase, alarm signal, problem alert, worry sign, alert expression, risk phrase, warning cue, danger phrase, concern signal
   Antonyms: reassurance, comfort sign, calming signal, positive feeling, confidence boost, safe expression, approval feeling, encouraging sign, trust expression, peace signal, relief sign, supportive message