The Collaborative International Dictionary of English v.0.48:

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]

The Collaborative International Dictionary of English v.0.48:

Basset horn \Bas"set horn`\ (b[a^]s"s[e^]t h[^o]rn`). [See
   Basset, a.] (Mus.)
   An instrument blown with a reed, and resembling a clarinet,
   but of much greater compass, embracing nearly four octaves;
   The corno di bassetto.
   [1913 Webster]

WordNet (r) 3.0 (2006):

basset horn
    n 1: a tenor clarinet; pitched in the key of F below the B-flat
         clarinet