torpedo - a small explosive device that is placed on a railroad track and fires when a train runs over it; the sound of the explosion warns the engineer of danger ahead

signal, signaling, sign - any communication that encodes a message; "signals from the boat suddenly stopped"

air alert - the warning signal that begins an period of preparation for an enemy air attack

burglar alarm - a loud warning signal produced by a burglar alarm; "they could hear the burglar alarm a mile away"

fire alarm - a shout or bell to warn that fire has broken out

foghorn, fogsignal - a loud low warning signal that can be heard by fog-bound ships

horn - a noise made by the driver of an automobile to give warning;

red flag - a flag that serves as a warning signal; "we didn't swim at the beach because the red flag was up"

siren - a warning signal that is a loud wailing sound

alarm bell, tocsin - the sound of an alarm (usually a bell)