A horn is any of a family of musical instruments made of a tube, usually made of metal and often curved in various ways, with one narrow end into which the musician blows, and a wide end from which sound emerges. In horns, unlike other brass instruments such as the trumpet, the bore gradually increases in width through most of its length—that is to say, it is conical rather than cylindrical. In jazz and popular-music contexts, the word may be used loosely to refer to any wind instrument, and a section of brass or woodwind instruments, or a mixture of the two, is called a horn section in these contexts.

The French horn (since the 1930s known simply as the "horn" in professional music circles) is a brass instrument made of tubing wrapped into a coil with a flared bell. The double horn in F/B♭ (technically a variety of German horn) is the horn most used by professional bands and orchestras. A musician who plays any kind of horn is called a horn player (or less frequently, a hornist).