1. An ancient wind instrument, originally made from animal horn, metal, or wood. In the Medieval and Renaissance eras, the horn was used to signal in battles and in hunts. The horn gradually evolved through a series of transformations: the horn for hunting became spiraled so its length would not interfere with its mobility, this natural horn became more refined and found its way into the early Classical orchestra as a character instrument, implying military or hunting scenes. Eventually crooks, and then valves were invented for the horn, enabling it to be fully chromatic.
The modern horn is an Alto range brass instrument with a conical bore and valves to change the length of the tubing, and thus the tones available to the instrument, making it fully chromatic from B1 below the bass clef staff to F5 on the top line of the treble clef staff. This is a transposing instrument in F, so those sounded pitches would be written a fifth higher. In America, it is often referred to as the French horn.
2. A slang term referring to any wind instrument.