The Collaborative International Dictionary of English v.0.48:

Alpenhorn \Al"pen*horn`\, Alphorn \Alp"horn`\, n. [G.
   Alpenhorn.]
   A curved wooden horn about three feet long, with a cupped
   mouthpiece and a bell, used by the Swiss to sound the ranz
   des vaches and other melodies. Its notes are open harmonics
   of the tube.
   [Webster 1913 Suppl.]