bass horn

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A large, valved, brass instrument with a bass pitch; a tuba or a sousaphone.
• noun An instrument similar to the serpent but shaped like a bassoon, used primarily in military bands in the early 19th century.

from the GNU version of the Collaborative International Dictionary of English.

• (Mus.) A modification of the bassoon, much deeper in tone.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A large brass instrument in the bass range, usually referring to the modern tuba or the archaic serpent

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the lowest brass wind instrument