When used alone, the word "horn" means either "hunting horn" or "French horn", depending on context. Other instruments are identified by specific adjectives such as "English horn" or "basset horn".
a loud audible alarm, especially one on a motor vehicle.
a conical device used to direct waves, such as an antenna horn or loudspeaker horn.
(Slang) generally, any brass wind instrument.
(Slang) telephone. Refers to the horn-shaped ends of old communication systems using air tubes.