A serpent is a wind instrument with a mouthpiece like a brass instrument but side holes like a woodwind instrument. It is a long cone bent into a snakelike shape, hence the name. The serpent is closely related to the cornett. It is generally made out of wood, with walnut being a particularly popular choice. Despite this and the fact that it has fingerholes rather than valves, it is usually classed as a brass, rather than a woodwind, instrument. The Hornbostel-Sachs scheme of musical instrument classification places it alongside trumpets.

On early models, the fingerholes were keyless, like ona recorder. Later models, however, add keys, as on a clarinet. The range varies according to the instrument and the player, but typically covers an octave either side of middle C. Some documents from the 18th century, however, state that the instrument can reach notes over two octaves above middle C.

It is thought that the instrument was first used to strengthen the sound of choirs in plainchant. Around the middle of the 18th century, it began to be used in military bands, but was replaced in the 19th century by valved brass instruments. Since then, it has hardly been used at all, although many original models still survive, and it is sometimes played as part of historically authentic performances.

A variation on the serpent was the bass horn, which is essentially the same, but is simpler in shape, consisting of a tube folded back on itself (rather like the modern bassoon), rather than the curvy shape of the original instrument.

A later variation was the ophimonocleide, a sort of cross between the bass horn and the ophicleide. It was never common, and today only a few examples of it exist.