Definition and background:
A tall, narrow bass brass instrument of the keyed bugle family invented in about 1817 in Paris by Jean-Hilaire Aste. The ophicleide has nine to twelve side keys in the manner of woodwind instruments. The bass ophicleide was the most successful instrument and was found in orchestras, bands, and opera orchestras through the 1840s.
For more information on the music term "Bass ophicleide" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary