What is "basset horn"

Corno di bassetto \Cor"no di bas*set"to\ (k?r"n? d? b?s-s?t"t? or b?s-s?t"t?); pl. Corni di basseto. [It.] (Mus.) A tenor clarinet; -- called also basset horn, and sometimes confounded with the English horn, which is a tenor oboe. [1913 Webster] ||

n. An alto instrument of the clarinet family, pitched in F below middle C, with a range reaching down to F below that.

n. a tenor clarinet; pitched in the key of F below the B-flat clarinet

The basset horn (sometimes written basset-horn) is a musical instrument, a member of the clarinet family.