A foghorn is a warning signal that makes a loud noise when it's very foggy. The sound of a foghorn is a warning to sailors and ship captains.

On foggy days near the coast, you might hear the repeated low, loud sound of a foghorn. Its warning noise lets those at sea know when they're approaching a rocky coastline or other obstacles they can't see in the fog. The earliest foghorns were gongs or bells that had to be manually hit, or in some cases cannons that were fired repeatedly as long as the fog lasted.

Definitions of foghorn

n a warning device consisting of a horn that generates a loud low tone

a foghorn that makes a signal consisting of two tones
Type of:
an alarm device that makes a loud warning sound
signaling device
a device used to send signals

n a loud low warning signal that can be heard by fogbound ships

Type of:
alarm, alarum, alert, warning signal
an automatic signal (usually a sound) warning of danger

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