Source : Webster's Revised Unabridged Dictionary (1913)

Basset horn \Bas"set horn`\ (Mus.)
   The corno di bassetto.

Basset horn \Bas"set horn`\ [See {Basset}, a.] (Mus.)
   An instrument blown with a reed, and resembling a clarinet,
   but of much greater compass, embracing nearly four octaves.

Source : WordNet®

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