Wiktionary
n. A bass instrument in the clarinet family.
WordNet
n. a large clarinet whose range is an octave below the B-flat clarinet
Wikipedia
The bass clarinet is a musical instrument of the clarinet family. Like the more common soprano B clarinet, it is usually pitched in B (meaning it is a transposing instrument on which a written C sounds as B), but it plays notes an octave below the soprano B clarinet. Bass clarinets in other keys, notably C and A, also exist, but are very rare (in contrast to the regular A clarinet, which is quite common in classical music). Bass clarinets regularly perform in orchestras, wind ensembles/concert bands, occasionally in marching bands, and play an occasional solo role in contemporary music and jazz in particular.
Someone who plays a bass clarinet is called a bass clarinetist.
Usage examples of "bass clarinet".
The cello seemed totally used for a simplified bass version of the melody, rather than for true harmony, as was the deep sound of the bass clarinet/English horn.
And, from time to time, helping some poor needy soul with a generous loan of $2 on a bass clarinet which cost him $85 in 1917, and which you can sell for $45 any time.