Red flag (idiom)

The United States Air Force refers to its largest annual exercise as Red Flag operation.

The results suggest that the RFC is effective in promoting prosocial bystander behavior, with recommendations provided for its implementation on college campuses.

By 1865, another law reduced speed limits even more and required someone to walk 60 yards ahead with a red flag to warn people and stop the engine if horses got scared.

A signal of danger or a problem can be referred to as a red flag, a usage that originated in the 18th century[citation needed].

The term "red flag" is used, e.g., during screening of communications, and refers to specific words or phrases encountered that might indicate relevance to the case.