yellow alert

noun

1. (in military or civilian defense) the first alert given when enemy aircraft are discovered approaching a military installation, city, coastline, etc. Compare blue alert, red alert, white alert.
2. the signal or alarm sounded for this alert.
3. any primary stage of alert, as when toxic chemicals have escaped in a particular area and a potential hazard exists.

Word History and Origins

Origin of yellow alert¹

First recorded in 1965–70