Wiktionary
n. a brass instrument in the baritone range that is similar to a euphonium
WordNet
n. the second lowest brass wind instrument [syn: baritone]
Wikipedia
The baritone horn, or sometimes just called baritone, is a low-pitched brass instrument. It is a piston-valve brass instrument with a bore that is part cylindrical and part conical, like the cornet and alto (tenor) horn. It uses a wide-rimmed cup mouthpiece like that of its peers, the trombone and euphonium. Like the trombone and the euphonium, the baritone can be considered either a transposing or non-transposing instrument.
In the UK the baritone is frequently found in brass bands. The baritone horn in the United States is common in school and university bands, the baritones found in school inventories often being older models as the instrument over time is yielding in popularity to the euphonium.
A person who plays a baritone horn is a baritone (horn) player or baritonist.