The horn on a car, bus, or train is the device that makes a loud honking sound to warn people or get their attention. Drivers use their horns to alert pedestrians crossing without looking, or to signal danger to other drivers. Ships have deep, booming horns that can be heard for miles across the water, warning other vessels in thick fog.

A horn is also a hard, pointed structure that grows from the heads of animals like cattle, goats, rhinoceroses, and antelope. Unlike antlers (which deer shed and regrow each year), horns are permanent. Animals use them for defense, establishing dominance, or fighting for territory. Some horns spiral magnificently, while others curve or jut straight out.

The word also refers to musical instruments made from animal horns or designed to look like them. French horns, used in orchestras, have a rich, mellow sound. Hunting horns were used for centuries to signal during hunts. Brass instruments as a group are often called horns.

When something is horn-shaped, it means curved and pointed like an animal's horn. The phrase to toot your own horn means to brag about yourself, while to lock horns means to get into a serious disagreement or fight with someone.