The Collaborative International Dictionary of English v.0.48:

Bombardo \Bom*bar"do\, Bombardon \Bom*bar"don\, n. [It.
   bombardo.] (Mus.)
   Originally, a deep-toned instrument of the oboe or bassoon
   family; thence, a bass reed stop on the organ. The name
   bombardon is now given to a brass instrument, the lowest of
   the saxhorns, in tone resembling the ophicleide. --Grove.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bombardon
    n 1: a tuba that coils over the shoulder of the musician [syn:
         helicon, bombardon]
    2: a large shawm; the bass member of the shawm family [syn:
       bombardon, bombard]