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.

Source : WordNet®

horseback
     n 1: the back of a horse
     2: a narrow ridge of hills [syn: {hogback}]
     adv : on the back of a horse; "he rode horseback to town";
           "managed to escape ahorse"; "policeman patrolled the
           streets ahorseback" [syn: {ahorse}, {ahorseback}]