red light
US /ˌred ˈlaɪt/
UK /ˌred ˈlaɪt/
1.
a signal light that is red, used as a warning or to indicate that a vehicle or pedestrian must stop
:
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The car stopped at the red light.
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Don't cross the street when the red light is on.
2.
a warning or sign of danger or prohibition
:
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The company's declining sales were a red light for investors.
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His sudden change in behavior was a red light that something was wrong.