The Collaborative International Dictionary of English v.0.48:

Cornet-a-piston \Cor"net-[`a]-pis`ton\ (k?r"n?t-?-p?s"t?n; F.
   k?r`n?`?p?s`t?n"), n.; pl. Cornets-[`a]-piston. [F.]
   (Mus.)
   A brass wind instrument, like the trumpet, furnished with
   valves moved by small pistons or sliding rods; a cornopean; a
   cornet.
   [1913 Webster]