Real Dictionary

horn
countable noun: On a vehicle such as a car, the horn is the device that makes a loud noise as a signal or warning.

Horn in American English
a hard, hollow, bony or keratinous, permanent projection that grows on the head of various hoofed animals, esp. bovid ruminants
Capecape on an island (Horn Island) in Tierra del Fuego, Chile: southernmost point of South America
noun: one of the bony, permanent, hollow paired growths, often curved and pointed, that project from the upper part of the head of certain ungulate mammals, as cattle, sheep, goats, or antelopes

horn in Automotive Engineering
noun: On a vehicle such as a car, the horn is the device that makes a loud noise as a signal or warning.

horn in British English
noun
noun: either of a pair of permanent outgrowths on the heads of cattle, antelopes, sheep, etc, consisting of a central bony core covered with layers of keratin