Definition of "warning signal" [warning signal]

• An automatic signal (usually a sound) warning of danger (noun)

WordNet. Princeton University. 2010.

• "This I afterward discovered was one of the secret signs of the San Luang; and the warning signal was addressed to me, because they imagined that I also was a member of the Council."
• "Suddenly he overturned the camp-bed, throwing Jojo on to the ground, for he thought that there might be an electric switch working a warning signal outside."
• "Brad glanced uneasily at Percey as he changed the transponder setting — to the code that automatically sent a warning signal to all radar facilities in the area that Foxtrot Bravo was in trouble."