Source : Webster's Revised Unabridged Dictionary (1913)

Horseback \Horse"back`\, n.
   1. The back of a horse.

   2. An extended ridge of sand, gravel, and bowlders, in a
      half-stratified condition. --Agassiz.

   {On horseback}, on the back of a horse; mounted or riding on
      a horse or horses; in the saddle.

            The long journey was to be performed on horseback.
                                                  --Prescott.